Introduction to Docker

Friday, 8 March 2019

Introduction to Docker

Docker is a platform which allows an application and all its dependencies to be packed in an image, shipped within this monolithic bundle and then deployed on any other machine with no fear that some dependency would be missing or not being compatible with the new host. Running version of the image is called a container.

Here are some terms:

host

may be local or remote

container

process which runs on a host

Docker runs processes in isolated containers.

Lifecycle of Docker Container

Image Source and Credits: http://docker-saigon.github.io/post/Docker-Internals/

image

Docker images can consist of multiple layers. Layers can be reused by images. If a layer is downloaded as part of imageA and it is also part of imageB then when we pull/download imageB, this layer will not be downloaded again but will be reused.

image ID is a SHA256 digest covering the image’s configuration and layers.

Benefits of containerisation

Standardized application packaging - no matter of the application's source language or framework, Docker images are created and applications packed in the same way
Multiplatform support - Docker image can be run on any OS, on a local machine or in the cloud
Containers are light-weight and isolated from each other

Applications running in Virtual Machines run on the following stack: Hardware - host OS - hypervisor - guest OS - software - application while those running in Docker run on: host OS - Docker Engine - Container - application. In case of VMs application can't utilize hardware to the full due to the fact that two OSes needs to run, on on top of another.
Docker images are also isolated, it is possible to use multiple containers in parallel; if one container has a failure, it will not affect other containers

Example:

running common development tools as Docker containers instead of installing them on the development host: Whats the use case for Terraform as a Docker Container? : r/Terraform

Docker Architecture

Docker installation on a local machine comprises of two components:

client - we issue commands (e.g. docker images, docker container ls, ...) to it and it forwards them to daemon
daemon

pulls/pushes images from/to image registry/repositories
manages local images
manages local containers

Dockerfile

Introduction to Dockerfile | My Public Notepad

Multi-stage build

This is one of the use cases which shows benefits of multi-staged build:
Docker container running golang http.Client getting error `certificate signed by unknown authority`

---

If we already have app binary built and can simply copy it onto an image but still have to add certificates, Dockerfile can look like:

FROM golang:alpine as build

RUN apk --no-cache add ca-certificates

FROM scratch

COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/

COPY ./bin/my_app /my_app

CMD ["/my_app"]

---
If we set working directory in the image created at first stage, it will remain set to the same path in the following images:

FROM python:3-alpine as base

# Required for installing psycopg2 (see https://github.com/psycopg/psycopg2/issues/684)
RUN apk update && apk add --no-cache postgresql-dev python3-dev gcc musl-dev

ENV appDir /usr/local/src/python-demo
WORKDIR ${appDir}

COPY ./requirements.txt ./
RUN pip install -r requirements.txt

FROM base
COPY . .
COPY src/ ./src
RUN pwd // /usr/local/src/python-demo
CMD [ "python", "./python_demo.py" ]

---

FROM scratch

If we use FROM scratch then we can't use/execute bash/sh terminal if we attach to the container from the outside. The lightest image which provides bash terminal is alpine:

FROM alpine

alpine contains /bin/sh so we can use it via

$ docker exec -it <container_name> sh

Attaching to a docker container with base image scratch?

docker-alpine redis how to get into bash

Why we needed to run apk add ca-certificates?
Install Certificates in Alpine Image to establish Secured Communication (SSL/TLS)

---

.dockerignore

added manually in the same directory where Dockerfile resides
a newline-separated list of patterns that define a set of files and/or directories that should be ignored by ADD and COPY commands in Dockerfile
should contain any directory/file present in the same directory as .dockerfile but which should not be in the newly built image e.g. application's output directory (present on the dev machine and populated during testing) which can be large in size. This can cause sending unnecessary files to Docker daemon and long times for building an image:

$ docker build -t my_image .

...

Sending build context to Docker daemon 20.66GB

.dockerignore Example:

# comment
.git
.gitignore
*/temp*
*/*/temp*
temp

# ignore all .md files apart from README.md

*.md
!README.md

Can dockerfile be put in .dockerignore?

Docker Tips : about the build context
Do not ignore .dockerignore (it's expensive and potentially dangerous) - Codefresh

Linter

hadolint

Example:

$ docker run --rm -i hadolint/hadolint < Dockerfile

Unable to find image 'hadolint/hadolint:latest' locally

latest: Pulling from hadolint/hadolint

447f1c00f9d8: Pull complete

Digest: sha256:6e67b08b2f9b3cf57616cfc92adb8864b74b02f079d70eabc944b29f05aff5f9

Status: Downloaded newer image for hadolint/hadolint:latest

/dev/stdin:27 DL3025 Use arguments JSON notation for CMD and ENTRYPOINT arguments

Docker CLI Commands

---

$ docker --help

Usage: docker [OPTIONS] COMMAND

A self-sufficient runtime for containers

Options:
--config string Location of client config files (default "/home/bojan/.docker")
-D, --debug Enable debug mode
-H, --host list Daemon socket(s) to connect to
-l, --log-level string Set the logging level ("debug"|"info"|"warn"|"error"|"fatal") (default "info")
--tls Use TLS; implied by --tlsverify
--tlscacert string Trust certs signed only by this CA (default "/home/bojan/.docker/ca.pem")
--tlscert string Path to TLS certificate file (default "/home/bojan/.docker/cert.pem")
--tlskey string Path to TLS key file (default "/home/bojan/.docker/key.pem")
--tlsverify Use TLS and verify the remote
-v, --version Print version information and quit

Management Commands:
builder Manage builds
config Manage Docker configs
container Manage containers
engine Manage the docker engine
image Manage images
network Manage networks
node Manage Swarm nodes
plugin Manage plugins
secret Manage Docker secrets
service Manage services
stack Manage Docker stacks
swarm Manage Swarm
system Manage Docker
trust Manage trust on Docker images
volume Manage volumes

Commands:
attach Attach local standard input, output, and error streams to a running container
build Build an image from a Dockerfile
commit Create a new image from a container's changes
cp Copy files/folders between a container and the local filesystem
create Create a new container
diff Inspect changes to files or directories on a container's filesystem
events Get real time events from the server
exec Run a command in a running container
export Export a container's filesystem as a tar archive
history Show the history of an image
images List images
import Import the contents from a tarball to create a filesystem image
info Display system-wide information
inspect Return low-level information on Docker objects
kill Kill one or more running containers
load Load an image from a tar archive or STDIN
login Log in to a Docker registry
logout Log out from a Docker registry
logs Fetch the logs of a container
pause Pause all processes within one or more containers
port List port mappings or a specific mapping for the container
ps List containers
pull Pull an image or a repository from a registry
push Push an image or a repository to a registry
rename Rename a container
restart Restart one or more containers
rm Remove one or more containers
rmi Remove one or more images
run Run a command in a new container
save Save one or more images to a tar archive (streamed to STDOUT by default)
search Search the Docker Hub for images
start Start one or more stopped containers
stats Display a live stream of container(s) resource usage statistics
stop Stop one or more running containers
tag Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE
top Display the running processes of a container
unpause Unpause all processes within one or more containers
update Update configuration of one or more containers
version Show the Docker version information
wait Block until one or more containers stop, then print their exit codes

Run 'docker COMMAND --help' for more information on a command.

Since version 1.13, Docker command-line has the following syntax:

docker <object> <command> <options>

In this syntax:

<object> indicates the type of Docker object you'll be manipulating. This can be a container, image, network or volume object.
<command> indicates the task to be carried out by the daemon, that is the run command.
<options> can be any valid parameter that can override the default behavior of the command, like the --publish option for port mapping.

docker build

Builds an image from a Dockerfile.

$ docker build --help
Usage: docker build [OPTIONS] PATH | URL | -
Build an image from a Dockerfile
Options:
--add-host list Add a custom host-to-IP mapping (host:ip)
--build-arg list Set build-time variables
--cache-from strings Images to consider as cache sources
--cgroup-parent string Optional parent cgroup for the container
--compress Compress the build context using gzip
--cpu-period int Limit the CPU CFS (Completely Fair Scheduler) period
--cpu-quota int Limit the CPU CFS (Completely Fair Scheduler) quota
-c, --cpu-shares int CPU shares (relative weight)
--cpuset-cpus string CPUs in which to allow execution (0-3, 0,1)
--cpuset-mems string MEMs in which to allow execution (0-3, 0,1)
--disable-content-trust Skip image verification (default true)
-f, --file string Name of the Dockerfile (Default is 'PATH/Dockerfile')
--force-rm Always remove intermediate containers
--iidfile string Write the image ID to the file
--isolation string Container isolation technology
--label list Set metadata for an image
-m, --memory bytes Memory limit
--memory-swap bytes Swap limit equal to memory plus swap: '-1' to enable unlimited swap
--network string Set the networking mode for the RUN instructions during build (default "default")
--no-cache Do not use cache when building the image
--pull Always attempt to pull a newer version of the image
-q, --quiet Suppress the build output and print image ID on success
--rm Remove intermediate containers after a successful build (default true)
--security-opt strings Security options
--shm-size bytes Size of /dev/shm
-t, --tag list Name and optionally a tag in the 'name:tag' format
--target string Set the target build stage to build.
--ulimit ulimit Ulimit options (default [])

The PATH specifies where to find the files for the “context” of the build on the Docker daemon.

Example (has to be run in a directory which contains Dockerfile):

$ docker build -t <my username>/<my-app-name> .

Example of how to build and run a Docker image:

$ docker build -t jsonschema2db . && docker run jsonschema2db

If Docker has to fetch some resources from remote host whose domain name can't be resolved from within Docker native network but can from your local/dev host network use --network host:

$ docker build --network host -t my-image .

--pull instructs docker to pull the latest version of any base image(s) instead of reusing whatever you already have tagged locally. From https://stackoverflow.com/questions/58488535/whats-the-purpose-of-docker-build-pull:

Take for instance an image based on a moving tag (such as ubuntu:bionic). Upstream makes changes and rebuilds this periodically but you might have a months old image locally. Docker will happily build against the old base. --pull will pull as a side effect so you build against the latest base image.

It's usually a best practice to use it to get upstream security fixes as soon as possible (instead of using stale, potentially vulnerable images). Though you have to trade off breaking changes (and if you use immutable tags then it doesn't make a difference).

--no-cache forces rebuilding of layers already available.

In normal use you shouldn’t need --no-cache. If the base image is updated (and --pull gets a new version) that automatically invalidates the cache; similarly if you COPY different code that will invalidate the cache. The only thing this will usually affect is if you’re doing something like apt-get install of a network-hosted package, in which case --no-cache will cause it to check for a newer version even if the base image hasn’t updated.

--build-arg arg=value - used to pass to Dockerfile an argument from command line. arg needs to be defined in Dockerfile via ARG command:

ARG arg[=default_value]

If arg is http_proxy or https_proxy there is no need to define these arguments as they are defined by default (predefined args).

---

docker container

Used for managing containers.

Exploring Docker commands: docker container | My Public Notepad

docker create

$ docker create --help

Usage: docker create [OPTIONS] IMAGE [COMMAND] [ARG...]

Create a new container

Options:
--add-host list Add a custom host-to-IP mapping (host:ip)
-a, --attach list Attach to STDIN, STDOUT or STDERR
--blkio-weight uint16 Block IO (relative weight), between 10 and 1000, or 0 to disable (default 0)
--blkio-weight-device list Block IO weight (relative device weight) (default [])
--cap-add list Add Linux capabilities
--cap-drop list Drop Linux capabilities
--cgroup-parent string Optional parent cgroup for the container
--cidfile string Write the container ID to the file
--cpu-period int Limit CPU CFS (Completely Fair Scheduler) period
--cpu-quota int Limit CPU CFS (Completely Fair Scheduler) quota
--cpu-rt-period int Limit CPU real-time period in microseconds
--cpu-rt-runtime int Limit CPU real-time runtime in microseconds
-c, --cpu-shares int CPU shares (relative weight)
--cpus decimal Number of CPUs
--cpuset-cpus string CPUs in which to allow execution (0-3, 0,1)
--cpuset-mems string MEMs in which to allow execution (0-3, 0,1)
--device list Add a host device to the container
--device-cgroup-rule list Add a rule to the cgroup allowed devices list
--device-read-bps list Limit read rate (bytes per second) from a device (default [])
--device-read-iops list Limit read rate (IO per second) from a device (default [])
--device-write-bps list Limit write rate (bytes per second) to a device (default [])
--device-write-iops list Limit write rate (IO per second) to a device (default [])
--disable-content-trust Skip image verification (default true)
--dns list Set custom DNS servers
--dns-option list Set DNS options
--dns-search list Set custom DNS search domains
--entrypoint string Overwrite the default ENTRYPOINT of the image
-e, --env list Set environment variables
--env-file list Read in a file of environment variables
--expose list Expose a port or a range of ports
--group-add list Add additional groups to join
--health-cmd string Command to run to check health
--health-interval duration Time between running the check (ms|s|m|h) (default 0s)
--health-retries int Consecutive failures needed to report unhealthy
--health-start-period duration Start period for the container to initialize before starting health-retries countdown (ms|s|m|h) (default 0s)
--health-timeout duration Maximum time to allow one check to run (ms|s|m|h) (default 0s)
--help Print usage

-h, --hostname string
Container host name

--init Run an init inside the container that forwards signals and reaps processes
-i, --interactive Keep STDIN open even if not attached
--ip string IPv4 address (e.g., 172.30.100.104)
--ip6 string IPv6 address (e.g., 2001:db8::33)
--ipc string IPC mode to use
--isolation string Container isolation technology
--kernel-memory bytes Kernel memory limit
-l, --label list Set meta data on a container
--label-file list Read in a line delimited file of labels
--link list Add link to another container
--link-local-ip list Container IPv4/IPv6 link-local addresses
--log-driver string Logging driver for the container
--log-opt list Log driver options
--mac-address string Container MAC address (e.g., 92:d0:c6:0a:29:33)
-m, --memory bytes Memory limit
--memory-reservation bytes Memory soft limit
--memory-swap bytes Swap limit equal to memory plus swap: '-1' to enable unlimited swap
--memory-swappiness int Tune container memory swappiness (0 to 100) (default -1)
--mount mount Attach a filesystem mount to the container
--name string Assign a name to the container
--network string Connect a container to a network (default "default")
--network-alias list Add network-scoped alias for the container
--no-healthcheck Disable any container-specified HEALTHCHECK
--oom-kill-disable Disable OOM Killer
--oom-score-adj int Tune host's OOM preferences (-1000 to 1000)
--pid string PID namespace to use
--pids-limit int Tune container pids limit (set -1 for unlimited)
--privileged Give extended privileges to this container
-p, --publish list Publish a container's port(s) to the host
-P, --publish-all Publish all exposed ports to random ports
--read-only Mount the container's root filesystem as read only
--restart string Restart policy to apply when a container exits (default "no")
--rm Automatically remove the container when it exits
--runtime string Runtime to use for this container
--security-opt list Security Options
--shm-size bytes Size of /dev/shm
--stop-signal string Signal to stop a container (default "SIGTERM")
--stop-timeout int Timeout (in seconds) to stop a container
--storage-opt list Storage driver options for the container
--sysctl map Sysctl options (default map[])
--tmpfs list Mount a tmpfs directory
-t, --tty Allocate a pseudo-TTY
--ulimit ulimit Ulimit options (default [])
-u, --user string Username or UID (format: <name|uid>[:<group|gid>])
--userns string User namespace to use
--uts string UTS namespace to use
-v, --volume list Bind mount a volume
--volume-driver string Optional volume driver for the container
--volumes-from list Mount volumes from the specified container(s)
-w, --workdir string Working directory inside the container

---

Examples:

$ docker create \

-e DB_HOST=121.1.0.1 \

-e DB_PORT=5432 \

-e DB_NAME=test_db \

-e DB_USER=postgres \

-e DB_PASSWORD=postgres \

-e OUTPUT_DIR=./data-vol \

--rm \

-it \

--mount type=bind,src="$(pwd)/data-vol",target=/data-vol \

--network=bridge \

--name my_app_test_container \

docker.example.com/bojan/myapp:build-28

If environment variable contains a question mark character which we want to preserve, this string has to be enclosed in single quotation mark.

Example: If we have:

docker run \
--rm \
-e NODE_ENV=dev \
-e API_BASE_URL=https://dev.example.com \
-e API_QUERY_TEMPLATE=?post_type={post_type}&json=1&count=1000 \
-e SCHEMAS_DIR=${SCHEMAS_DIR} \
-e PGHOST=postgres \
-e PGDATABASE=test_dev \
-e PGUSER=postgres \
-e PGPASSWORD=postgres \
-v "$(pwd)/${SCHEMAS_DIR}":/usr/local/my-app/${SCHEMAS_DIR} \
--name my-app \
--network myappnet \
--user $(id -u):$(id -g) \
example.com/docker/myapp:build-%docker-image.build.wpsync%

it would fail with error:

-e: not found

Because docker run would interpret/expand ? as wildcard (?). Fix is to enclose env var value in single quotes:

docker run \
...
-e API_QUERY_TEMPLATE='?post_type={post_type}&json=1&count=1000' \
...

shell - How to escape "!" and "&" in docker's environment varibles - Super User

docker exec

To execute a command inside a running container, use docker exec. For example, we can run a bash terminal as:

$ docker exec -it my-postgres bash
root@a4a7486fea59:/#

-i // keeps stdin opened
-t // allocates pseudo TTY

In this terminal we can run usual Linux commands as we were inside the container:

root@a4a7486fea59:/# ls
bin boot dev docker-entrypoint-initdb.d docker-entrypoint.sh etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
root@a4a7486fea59:/# pwd
/

not able to use docker exec shell
Try the alpine tag of image:

$ docker run --name gorush -d appleboy/gorush:alpine
$ docker exec -ti gorush /bin/sh

docker inspect

$ docker inspect --help

Usage: docker inspect [OPTIONS] NAME|ID [NAME|ID...]

Return low-level information on Docker objects

Options:
-f, --format string Format the output using the given Go template
-s, --size Display total file sizes if the type is container
--type string Return JSON for specified type

Examples:

How to find mounting points of a (stopped) container? E.g. find local directory that is mounted as a volume on some container.

$ docker inspect -f {{.Mounts}} my_container

[{bind /home/bojan/tmp/data-vol /data-vol true rprivate}]

What to Inspect When You're Inspecting

docker logs

Exploring Docker commands: docker logs | My Public Notepad

docker network

$ docker network --help

Usage: docker network COMMAND

Manage networks

Commands:
connect Connect a container to a network
create Create a network
disconnect Disconnect a container from a network
inspect Display detailed information on one or more networks
ls List networks
prune Remove all unused networks
rm Remove one or more networks

Run 'docker network COMMAND --help' for more information on a command.

List all networks:

$ docker network ls

NETWORK ID NAME DRIVER SCOPE

110d23a9b23f bridge bridge local

b32aba0ecc85 my_app1 bridge local

5a91d6b95b25 my_app2 bridge local

7422e6165f7a my_app3 bridge local

b2977102daa4 my_app4_test-network bridge local

e829cad5a344 host host local

6b0d605ed4a5 none null local

Connect a container to the network (of some other container):

$ docker network connect --help

Usage: docker network connect [OPTIONS] NETWORK CONTAINER

Connect a container to a network

Options:

--alias strings Add network-scoped alias for the container

--ip string IPv4 address (e.g., 172.30.100.104)

--ip6 string IPv6 address (e.g., 2001:db8::33)

--link list Add link to another container

--link-local-ip strings Add a link-local address for the container

Use case example:

Running Postgres DB in one container (based on postgres image; container runs in its own network) and connecting to it via pgAdmin which runs in another container (based on dpage/pgadmin4 image).

docker container ls shows that my_app_db_1 is up and running with port mapping as 0.0.0.0:5432->5432/tcp. This container runs Postgres DB.

To find the name of the network this container is connected to use docker inspect <container_name> and in the output json look for "NetworkSettings" > "Networks". There should be a network name like "my_app_default".

Let's inspect that network:

$ docker network inspect my_app_default

[

{

"Name": "my_app_default",

"Id": "5a91d6b95b25fdd6d24e921e1ccdb69f24ae4b1c509188ccfc087acd330375ca",

"Created": "2019-06-05T10:19:11.576495786+01:00",

"Scope": "local",

"Driver": "bridge",

"EnableIPv6": false,

"IPAM": {

"Driver": "default",

"Options": null,

"Config": [

{

"Subnet": "172.19.0.0/16",

"Gateway": "172.19.0.1"

}

]

"Internal": false,

"Attachable": true,

"Ingress": false,

"ConfigFrom": {

"Network": ""

"ConfigOnly": false,

"Containers": {

"9ae5dfe822348906d2bdf2b55314a323f7ba03360eb2edf8e5d7d42afca40e34": {

"Name": "my_app_service",

"EndpointID": "a5230696caabc7d230df8f589066b8028c3b03493a2f05ed0dc5e224944952fc",

"MacAddress": "02:42:ac:13:00:03",

"IPv4Address": "172.19.0.3/16",

"IPv6Address": ""

"e52e5d6de6b6a062d2cbfaf921f3365f20db0208511a906cab6143528a14ee6d": {

"Name": "my_app_db_1",

"EndpointID": "bbe87692635abe7cd45a3980d0982ca1dbc6c9a5718a267935659cfb036b2da1",

"MacAddress": "02:42:ac:13:00:02",

"IPv4Address": "172.19.0.2/16",

"IPv6Address": ""

}

"Options": {},

"Labels": {

"com.docker.compose.network": "default",

"com.docker.compose.project": "my_app",

"com.docker.compose.version": "1.23.2"

}

]

my_app_db_1 is the name of the container which is running db service (Postgres) as I specified in docker-compose.yml.

Now, I want to run pgAdmin DB client (which is also a web server...) from its own container, based on dpage/pgadmin4 image:

$ docker run -p 5051:5051 -d -e "PGADMIN_DEFAULT_EMAIL=bojan.komazec@example.com" -e "PGADMIN_DEFAULT_PASSWORD=postgres" -e "PGADMIN_LISTEN_PORT=5051" --name pgadmin dpage/pgadmin4

baf9343368f9cb20a0dd9ab1a4763eaf08df03f2ee667f979b4e3466bc20d7b1

We can use docker container ls to verify that this container is now up and running, with port mappings as 80/tcp, 443/tcp, 0.0.0.0:5051->5051/tcp. We can now open in browser on the local host the address http://localhost:5051/ which should open pgAdmin login page. If we try to add DB server with IP address 172.19.0.2 (as listed in the postgres container network spec above), connecting to it will fail as pgAdmin runs in its own network. We need to connect pgadmin container to my_app_default network:

$ docker network connect my_app_default pgadmin

We can now use 172.19.0.2 address to connect to DB from pgAdmin web app.

If we again inspect my_app_default network we can verify that there is one more container attached to it: pgadmin.

To avoid issuing two commands, we can specify network container shall connect to in docker run command:

$ docker run -p 5051:5051 -d -e "PGADMIN_DEFAULT_EMAIL=bojan.komazec@example.com" -e "PGADMIN_DEFAULT_PASSWORD=postgres" -e "PGADMIN_LISTEN_PORT=5051" --network=my_app_default --name pgadmin dpage/pgadmin4

Creating a network:

$ docker network create --help

Usage: docker network create [OPTIONS] NETWORK

Create a network

Options:
--attachable Enable manual container attachment
--aux-address map Auxiliary IPv4 or IPv6 addresses used by Network driver (default map[])
--config-from string The network from which copying the configuration
--config-only Create a configuration only network
-d, --driver string Driver to manage the Network (default "bridge")
--gateway strings IPv4 or IPv6 Gateway for the master subnet
--ingress Create swarm routing-mesh network
--internal Restrict external access to the network
--ip-range strings Allocate container ip from a sub-range
--ipam-driver string IP Address Management Driver (default "default")
--ipam-opt map Set IPAM driver specific options (default map[])
--ipv6 Enable IPv6 networking
--label list Set metadata on a network
-o, --opt map Set driver specific options (default map[])
--scope string Control the network's scope
--subnet strings Subnet in CIDR format that represents a network segment

Example:

Create a network:

$ docker network create --subnet=172.18.0.0/24 --gateway=172.18.0.1 mynet123

Attach container to that network and assign a static IP address to it:

$ docker run --net mynet123 --ip 172.18.0.22 -it ubuntu bash

Assign static IP to Docker container

Destroying a network

$ docker network rm mynet123

---

docker ps

To list all running containers use docker ps:

$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
b9f263f2b517 postgres "some.command…" 9 hours ago Up 5 hours 0.0.0.0:5432->5432/tcp my_project_postgres_1

If no containers are running:

$ sudo docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

To list all containers, use docker ps -a.

docker pull

docker pull pulls an image from the repository.

Use:

$ docker pull <image_name>:<tag_name>

If no tag is provided, Docker Engine uses the :latest tag as a default.

Apart from pulling some image for the first time, this command is used to pull a new version of the existing image. [Update a docker container to the latest version]

In my case, I have been running pgAdmin4 from within a container but after some time I started getting a popup on pgAdmin4 main web page that I am using the old version of it. My course of actions was:

$ docker images | grep pgadmin
dpage/pgadmin4 latest c32b95c07a00 5 weeks ago 237MB

$ docker pull dpage/pgadmin4
Using default tag: latest
latest: Pulling from dpage/pgadmin4
e7c96db7181b: Already exists
...
44bc3cf004be: Already exists
aa774cf9fd8b: Pull complete
...
660e231737f7: Pull complete
Digest: sha256:2c46b3e631f33434246a2a51134e3a43951b00c5639f7fc158b4b973879f9ea3
Status: Downloaded newer image for dpage/pgadmin4:latest

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How to upgrade docker container after its image changed
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docker run

Exploring Docker commands: docker run | My Public Notepad

docker search

Search for images available at Docker Hub.

% docker search mysql

NAME DESCRIPTION STARS OFFICIAL AUTOMATED

mysql MySQL is a widely used, open-source relation… 13600 [OK]

mariadb MariaDB Server is a high performing open sou… 5188 [OK]

phpmyadmin phpMyAdmin - A web interface for MySQL and M… 707 [OK]

percona Percona Server is a fork of the MySQL relati… 597 [OK]

bitnami/mysql Bitnami MySQL Docker Image 80 [OK]

databack/mysql-backup Back up mysql databases to... anywhere! 77

linuxserver/mysql-workbench 45

ubuntu/mysql MySQL open source fast, stable, multi-thread… 40

linuxserver/mysql A Mysql container, brought to you by LinuxSe… 37

circleci/mysql MySQL is a widely used, open-source relation… 28

google/mysql MySQL server for Google Compute Engine 22 [OK]

docker start

To start a container use docker start. In the following example container name is my-postgres:

$ docker start my-postgres
my-postgres

docker stop

To stop running one or more containers use docker stop:

$ docker stop my_service_postgres_1 my-other-app
my_service_postgres_1
my-other-app

docker image

Exploring Docker commands: docker image | My Public Notepad

docker images

Exploring Docker commands: docker images | My Public Notepad

docker volume

$ docker volume

Usage: docker volume COMMAND

Manage volumes

Commands:
create Create a volume
inspect Display detailed information on one or more volumes
ls List volumes
prune Remove all unused local volumes
rm Remove one or more volumes

Run 'docker volume COMMAND --help' for more information on a command.

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docker volume ls

$ docker volume ls --help

Usage: docker volume ls [OPTIONS]

List volumes

Aliases:
ls, list

Options:
-f, --filter filter Provide filter values (e.g. 'dangling=true')
--format string Pretty-print volumes using a Go template
-q, --quiet Only display volume names

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$ docker volume ls
DRIVER VOLUME NAME
local 1a2a9d0f43ce94ad1e176f9bb318f46829fcd34354d4d648d70434efa0f43f60
local 1a66b905bcd4be936b8df26cacf018d7f863e758773a11be5faf9d9949311eb0
local 1c6af16b8d8b1041ad3a46e3910bde9121b64cfae65f8299cedd81793d8fce37
...

$ docker volume ls -q
1a2a9d0f43ce94ad1e176f9bb318f46829fcd34354d4d648d70434efa0f43f60
1a66b905bcd4be936b8df26cacf018d7f863e758773a11be5faf9d9949311eb0
1c6af16b8d8b1041ad3a46e3910bde9121b64cfae65f8299cedd81793d8fce37

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docker volume rm

$ docker volume rm --help

Usage: docker volume rm [OPTIONS] VOLUME [VOLUME...]

Remove one or more volumes

Aliases:
rm, remove

Examples:

$ docker volume rm hello
hello

Options:
-f, --force Force the removal of one or more volumes
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To remover all volumes:

$ docker volume rm $(docker volume ls -q)

TBC...