Docker

Docker is a set of platform as a service (PaaS) products that use OS-level virtualisation to deliver software in packages called containers

• The software that hosts the containers is called Docker Engine

Example:

# pull a docker image
docker pull alpine

# run a new container from the image
docker run -t -d --name <customName> alpine

# check currently running Docker containers
docker ps

# connect to a container
docker exec -it <customName> bash

# stop a container
docker stop <customName>

# check container stats
docker stats

Container

A way to package application with all the necessary dependencies and configuration

• Portable artifact, easily shared and moved around.

Need of containers:

• Compatibility / Dependency
• Long setup time
• Different Dev/Test/Prod environments

Installation

• One click installation script:

curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh

• Run docker as non root user:

sudo groupadd docker
sudo usermod -aG docker $USER

• Install Docker Engine

Docker CLI

Example:

docker build .

docker build -t <image-name>:<version> .

docker image ls

docker image rm <image-id>
docker rmi -f <image-id>

docker run -p 8080:3000 -d --name <container-name> <image-name>

docker run -it <container-name>

# enter the container
docker exec -it <container-name> bash

# delete container
docker rm <container-name> -f

• Run Containers:

Command	Description
docker run IMAGE	Start a new container
docker run --name CONTAINER IMAGE	Start a new container and set a name
docker run -p HOSTPORT:CONTAINERPORT IMAGE	Start a new container with mapped ports
docker run -P IMAGE	Start a new container and map all ports

• Container Management:

Command	Description
docker create IMAGE	Create a new container
docker start CONTAINER	Start a container
docker stop CONTAINER	Graceful stop a container
docker kill CONTAINER	Kill (SIGKILL) a container
docker restart CONTAINER	Graceful stop and restart a container
docker pause CONTAINER	Suspend a container
docker unpause CONTAINER	Resume a container
docker rm CONTAINER	Destroy a container

• Container Bulk Management:

Command	Description
docker stop $(docker ps -q)	To stop all the running containers
docker stop $(docker ps -a -q)	To stop all the stopped and running containers
docker kill $(docker ps -q)	To kill all the running containers
docker kill $(docker ps -a -q)	To kill all the stopped and running containers
docker restart $(docker ps -q)	To restart all running containers
docker restart $(docker ps -a -q)	To restart all the stopped and running containers
docker rm $(docker ps -q)	To destroy all running containers
docker rm $(docker ps -a -q)	To destroy all the stopped and running containers
docker pause $(docker ps -q)	To pause all running containers
docker pause $(docker ps -a -q)	To pause all the stopped and running containers
docker start $(docker ps -q)	To start all running containers
docker start $(docker ps -a -q)	To start all the stopped and running containers
docker rm -vf $(docker ps -a -q)	To delete all containers including its volumes use
docker rmi -f $(docker images -a -q)	To delete all the images
docker system prune	To delete all dangling and unused images, containers, cache and volumes
docker system prune -a	To delete all used and unused images
docker system prune --volumes	To delete all docker volumes

• Inspect Containers:

Command	Description
docker ps	List running containers
docker ps -a	List all containers, including stopped
docker logs CONTAINER	Show a container output
docker logs -f CONTAINER	Follow a container output
docker top CONTAINER	List the processes running in a container
docker diff	Show the differences with the image (modified files)
docker inspect	Show information of a container (json formatted)

• Run Commands:

Command	Description
docker attach CONTAINER	Attach to a container
docker cp CONTAINER:PATH HOSTPATH	Copy files from the container
docker cp HOSTPATH CONTAINER:PATH	Copy files into the container
docker export CONTAINER	Export the content of the container (tar archive)
docker exec CONTAINER	Run a command inside a container
docker exec -it CONTAINER /bin/bash	Open an interactive shell inside a container (there is no bash in some images, use /bin/sh)
docker wait CONTAINER	Wait until the container terminates and return the exit code

• Images:

Command	Description
docker images	List all local images
docker history IMAGE	Show the image history
docker inspect IMAGE	Show information (json formatted)
docker tag IMAGE TAG	Tag an image
docker commit CONTAINER IMAGE	Create an image (from a container)
docker import URL	Create an image (from a tarball)
docker rmi IMAGE	Delete images
docker pull REPO:[TAG]	pull an image/repo from a registry
docker push REPO:[TAG]	push and image/repo to a registry
docker search TEXT	Search an image on the official registry
docker login	Login to a registry
docker logout	Logout from a registry
docker save REPO:[TAG]	Export an image/repo as a tarball
docker load	Load images from a tarball

• Volumes:

Command	Description
docker volume ls	List all volumes
docker volume create VOLUME	Create a volume
docker volume inspect VOLUME	Show information (json formatted)
docker volume rm VOLUME	Destroy a volume
docker volume ls --filter="dangling=true"	List all dangling volumes (not referenced by any container)
docker volume prune	Delete all volumes (not referenced by any container)

Dockerfile

Each step is considered as a layer of an image

• Docker caches results layer by layer

Dockerfile statements:

Command	Purpose
FROM	To specify the parent image
WORKDIR	To set the working directory for any commands that follow in the Dockerfile
RUN	To install any applications and packages required for your container
COPY	To copy over files or directories from a specific location
ADD	As COPY, but also able to handle remote URLs and unpack compressed files
CNTRYPOINT	The command that will always be executed when the container starts. if not specified, the default is /bin/sh -c
CMD	Arguments passed to the entrypoint. If ENTRYPOINT is not set (defaults to /bin/sh -c) the CMD will be the commands the container executes
EXPOSE	To define which port through which to access your container application
LABEL	To add metadata to the image

Example:

• From scratch:

  # syntax=docker/dockerfile:1
  FROM scratch
  COPY helloworld /
  CMD ["/helloworld"]

• Python app:

  # Docker image
  FROM python:latest
  
  # Set the working directory
  WORKDIR /app
  
  # Install dependencies
  COPY ./requirements.txt /app
  RUN pip install --no-cache-dir --upgrade -r requirements.txt
  
  # Copy the scripts to the folder
  COPY . /app
  
  # Start the app
  CMD ["python main.py"]

.dockerignore file is used to prevent sensitive or unnecessary files and directories from making their way into your image builds

• It should be in the root directory, known as the build context

• ASP.NET Core app

Docker-Compose

Docker-compose is a tool that is used for multi-container applications in a single host

Getting started

Example: docker-compose.yaml file

version: "3.9"
services:
  web:
    build: .
    ports:
      - "8000:5000"
  redis:
    image: "redis:alpine"

Networking:

By default Docker-Compose will create a new network for the given compose file. You can change the behaviour by defining custom networks in your compose file

• Create and assign custom network:

Example: docker-compose.yml

networks:
  custom-network:
services:
  app:
    networks:
      - custom-network

• Use existing networks:

If you want to use an existing Docker network for your compose files, you can add the external: true parameter in your compose file

Example: docker-compose.yml

networks:
  existing-network:
    external: true

Volumes:

Volumes allow Docker containers to use persistent storage. In a compose file, you can create and map volumes like this:

volumes:
  my-volume:
services:
  app:
    volumes:
      - my-volume:/path-in-container

These volumes are stored in /var/lib/docker/volumes

Docker-Compose Commands

# create and start containers
docker-compose up

# start services with detached mode
docker-compose -d up

# start specific service
docker-compose up <service-name>

# list images
docker-compose images

# list containers
docker-compose ps

# start service
docker-compose start

# stop services
docker-compose stop

# display running containers
docker-compose top

# kill services
docker-compose kill

# remove stopped containers
docker-compose rm

# stop all containers and remove images, volumes
docker-compose down

Networking

Troubleshooting: docker run --name netshoot --rm -it nicolaka/netshoot /bin/bash

Backup and Restore Containers

Backup docker data from inside container volumes and package it in a tarball archive

docker run --rm --volumes-from CONTAINER -v $(pwd):/backup busybox tar cvfz /backup/backup.tar CONTAINERPATH

An automated backup can be done also by this Ansible playbook

The output is also a (compressed) tar. The playbook can also manage the backup retention. So older backups will get deleted automatically

To also create and backup the container configuration itself, you can use docker-replayfor that. If you lose the entire container, you can recreate it with the export from docker-replay

A more detailed tutorial on how to use docker-replay can be found here

Restore container from backup

Restore the volume with a tarball archive:

docker run --rm --volumes-from CONTAINER -v $(pwd):/backup busybox sh -c "cd CONTAINERPATH && tar xvf /backup/backup.tar --strip 1"

Reference

• Docker Homepage
• Docker Documentation
• "Learning Docker" by Arthur Ulfeldt
• Docker for .NET Developers with Visual Studio by Lee Brandt