Docker

This page covers the installation of Docker on Debian and a quickstart with compose.
We will assume your Debian machine is a blank state configured with the fastest mirrors.
It also contains a crash course to basic commands.

Installing Docker

Everything there needs to be run as room (you can use sudo for most commands).
Update your system with apt update and then install prerequisites apt install ca-certificates curl.

You can now begin:

sudo install -m 0755 -d /etc/apt/keyrings
sudo curl -fsSL https://download.docker.com/linux/debian/gpg -o /etc/apt/keyrings/docker.asc
sudo chmod a+r /etc/apt/keyrings/docker.asc
echo \
  "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian \
  $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
  sudo tee /etc/apt/sources.list.d/docker.list > /dev/null

Now, update and install Docker itself apt update; apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
This will install the base of Docker but also the docker-compose plugin that we will use later.

Now enable and start Docker and you’re all set:

systemctl enable docker
systemctl start docker

Basic Docker commands

Docker is extremely easy to use and contains a few basic commands:

docker pull <image> - Will pull an image (if no specified repository, it will pull it from the default hub.docker.com)
docker run <image> <options/arguments> - Will run the specified image (and pull it if not already done). This command accepts a few parameters (for the most popular):
- -p outsideport:insideport - Allows to map a port from the inside of the container to the outside, for instance -p 55:80 will make the port 80 of inside the container as available as port 50 externally
- -i - Starts the container in interactive mode, which will keep STDIN open even when not attached
- -t - Allocates a pseudo-TTY, basically attaches a console inside the container
- -e envvar=value - Sets an environment variable inside the container
- -d - Detach from the container, making it run in the background`
docker container ls -a - Lists all containers
docker container stop <container id> - Will stop the container
docker container rm <container id> - Will remove the container
docker container start <container id> - Will start the container
docker system prune -a - Will clean: all unused images, all unused containers and all unused networks
docker attach <container id> - Allows you to attach a TTY inside of the specified container

Docker Compose

Docker Compose is a tool that allows defining and running multiple containers from a configuration file written in YAML.
Docker Compose makes setting up services painless by automating much of the process.

For instance, let’s create a simple docker-compose.yml file for starters:

version: "3.9"

services:
  hello:
    image: hello-world

Now, if you issue docker compose up, you should see that the image is automatically pulled and run, showing the “Hello from Docker” message.
The docker compose command also has some basic parameters:

up <service> - Will bring the specified service up, note that if you don’t specify a service, Compose will bring all services in the configuration file up. You will also need to add the -d argument to that command to not be attached to the containers when they are started
stop <service> - Will stop the specified service
down - Will bring down all the services specified in the Compose configuration file
exec <service> <command> - Will execute the specified command in the container of the specified service
logs <service> - Allows you to see the logs of a specified service, if none is specified, it will show the logs of all the services. Adding -f to it will follow the log like with the tail -f command

One good thing about a compose file is that it will create a network for you, without having to link containers between them.
Let’s take for instance, a service with a database.

version: "3.9"

services:
  database:
    image: postgres:15
    volumes:
      - ./database:/var/lib/postgresql/data
    environment:
      POSTGRES_DB: "testdb"
      POSTGRES_USER: "test"
      POSTGRES_PASSWORD: "iamverysecret"

  service:
    image: mydatabaseservice
    environment:
      DB_HOST: "database"
      DB_USER: "test"
      DB_PASSWORD: "iamverysecret"
      DB_DB: "testdb"

Note: Trying to bring that Compose configuration up will fail as the service mydatabaseservice is only an example and doesn’t actually exists.

In that example, the service would be able to resolve the database and connect to it without having to link containers as a network would be created for this Compose file.
You can also see a demonstration of environment variables, though it is recommended to use a .env file and ENVIRONMENT_VARIABLE: ${VARIABLE_VALUE} in the Compose configuration file itself.