Docker¶

Docker is a tool to run applications in containers. A container is a lightweight virtual machine. It is a virtual machine because it is isolated from the host system. It is lightweight because it does not need a full operating system. Instead, it uses the host system's kernel. This makes it very fast and efficient. Docker is a tool to manage containers. It is also used by many developers to run their applications locally.

How it works¶

The applications run in so-called containers. These containers be descendend from images. Images are the blueprints for containers. These images get generated from Dockerfiles, which are the blueprints for images. You can create your own Dockerfiles to create your own images to run your own containers.

There are also pre-made images available on Docker Hub. These images are created by the community and can be used by everyone.

When you run a container from a pre-made image, it will be downloaded from the Registry (e.g. Docker Hub) and run on your machine. The Registry is a place where images are stored. You can also create your own Registry to store your own images or upload your own images to Docker Hub.

You can define multiple containers in a docker-compose.yml file. This way, you can run multiple containers at once. Then you can also create dependencies between the containers. For example, you can create a container for a database and a container for a web server. Then you create a dependency between them, so that the web server container will only start when the database container is running.

graph TD
    A[Dockerfile] --> B[Image]
    B --> C[Web Container]
    D[Registry download] --> E[Image]
    E --> F[DB Container]
    C --> G[Compose]
    F --> G
    G --> H[Web Container]
    G --> I[DB Container]
    H o--o I
    H --> J[Port 80]
    I --> K[Port 3306]

Getting Started¶

You can run Docker on your local machine (via Docker Desktop) or on a server (via CLI).

You can find a tutorial on how to get started with Docker here.

Docker Desktop¶

Docker Desktop is the easiest way to get started with Docker. It is available for Windows and macOS. You can download it here.

CLI¶

If you want to run Docker on a server, you can use it via the CLI¹. You can install it on Ubuntu with the following command:

sudo apt install docker.io

Host to Container mapping¶

When running a container, you can map ports and volumes from the host to the container.

Its always in the format <host>:<container>. For example, if you want to map port 8080 on the host to port 80 in the container, you would use 8080:80.

Ports¶

For example you want to run docker run -p 8080:80 <image>. This means that you want to run a container from the image <image> and map port 80 in the container to port 8080 on the host. This means that you can access the container via localhost:8080.

That means you can run multiple containers (e.g. 4 web servers) on the same host, but map them to different ports (e.g. 8080, 8081, 8082, 8083). This way, you can access all of them via the different port (e.g. localhost:8080, localhost:8081, localhost:8082, localhost:8083

docker run -p 8080:80 nginx
docker run -p 8081:80 nginx
docker run -p 8082:80 nginx
docker run -p 8083:80 nginx

Volumes¶

As you might know, containers are ephemeral. This means that all data in the container is lost when the container is stopped. To persist data, you can map a volume from the host to the container.

You can also use this to inject data into the container. For example, you can map a volume from the host to the container and put a file in it. This way, the file will be available in the container.

Example¶

docker run -v /some/content:/usr/share/nginx/html -d nginx

In this example, the content of /some/content on the host will be available in the container at /usr/share/nginx/html.

So when you create a file /some/content/index.html with the content Hello World!, you can access it in the container at localhost:8080/index.html.

Dockerfile¶

FROM ubuntu:latest

Docker Compose¶

Docker Compose is a tool to run multiple containers at once. You can define all containers in a docker-compose.yml file. You can find a tutorial on how to get started with Docker Compose here.

Commands¶

Some basic commands to get started with Docker:

Container Infos¶

docker ps # List running containers
docker ps -a # List all containers (also stopped/failed ones)
docker logs <container> # Show logs of a container (useful for debugging)

Run¶

docker run <image> # Run a container
docker run -it <image> # Run a container and open a shell
docker run -d <image> # Run a container in the background
docker run -p <host-port>:<container-port> <image> # Run a container and map a port
docker run -v <host-path>:<container-path> <image> # Run a container and map a volume

Stop¶

docker stop <container> # Stop a container
docker stop $(docker ps -a -q) # Stop all containers

Remove¶

docker rm <container> # Remove a container
docker rm $(docker ps -a -q) # Remove all containers

Prune¶

Docker stores all images and containers on the host. This can take up a lot of space. To free up space, you can use the prune command.

docker system prune # Remove all stopped containers, all dangling images, and all unused networks
docker system prune -a # Remove all stopped containers, all unused images, and all unused networks
docker system prune -a --volumes # Remove all stopped containers, all unused images, all unused networks, and all unused volumes

Compose¶

docker-compose up # Run all containers defined in docker-compose.yml
docker-compose up -d # Run all containers defined in docker-compose.yml in the background
docker-compose down # Stop all containers defined in docker-compose.yml
docker-compose down -v # Stop all containers defined in docker-compose.yml and remove all volumes

docker-compose logs # Show logs of all containers defined in docker-compose.yml
docker-compose logs <container> # Show logs of a container defined in docker-compose.yml
docker-compose logs -f # Show logs of all containers defined in docker-compose.yml and follow the output

Networking¶

Please watch the following video to learn more about Docker networking:

Docker Networking

Full Tutorial¶

You can find a full tutorial on how to get started with Docker here.

IT-Lab Collection¶

Refer to our Docker-Collection for a collection of Dockerfiles and docker-compose files.