What is Docker?

Docker is a free software used for container virtualization. This allows applications to run in containers, isolated and independent of the host environment. This simplifies the deployment of applications, as all necessary components such as code, runtime, system tools, and libraries are contained in the container and do not need to be installed on the other system.

Docker Terminology

When working with Docker, it is important to know some basic terms:

  • Image: A Docker image is an immutable template that contains all the necessary instructions to run an application. It is based on a series of layers, each representing changes from the previous layer. Images are created from Dockerfiles and can be stored in registries and retrieved from there.

  • Container: A container is a running instance of a Docker image. It contains everything the application needs to run in an isolated environment, including the code, runtime, system tools, and libraries. Containers are lightweight and portable, making them ideal for developing and deploying applications.

  • Dockerfile: A Dockerfile is a text file that contains a series of instructions on how to create a Docker image. Each instruction in a Dockerfile creates a new layer in the image. Typical instructions include FROM to specify the base image, RUN to execute commands, and COPY to copy files into the image.

  • Registry: A registry is a storage location for Docker images. The most well-known registry is Docker Hub, but there are also private registries that can be used within organizations. Registries allow images to be shared and versioned. Developers can push images to a registry and pull them from there to use on different systems.

  • Volume: A volume is a mechanism to store data outside the container so that it persists even after the container is stopped or deleted. Volumes are particularly useful for storing data that needs to be shared between multiple containers or for storing data that needs to persist across container restarts.

  • Network: Docker networks enable communication between containers and other services. There are different network modes, such as bridge, host, and overlay. The bridge mode is the default mode and creates a private internal network for containers on the same host. The host mode allows containers to have direct access to the host’s network. The overlay mode is used to connect containers across multiple hosts.

Example

To show the basic usage of Docker, let’s create a simple web server using the nginx webserver and run everything in a Docker container.

For this we can create a project directory which contains an HTML file (our website), as well as a Dockerfile which is needed to create our Docker image.

Here an example of a simple HTML website in the file index.html:

  <!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>My Docker Website</title>
  </head>
  <body>
    <h1>Welcome to my Docker website!</h1>
    <p>This is a simple static website, deployed using docker.</p>
  </body>
</html>

Now we need to create a Dockerfile which serves our HTML file with a webserver:

  # Using the nginx webserver as base image
FROM nginx:alpine

# Copy the index.html file to the html directory served by nginx
COPY index.html /usr/share/nginx/html/index.html

# Expose the Port so that it can be accessed
EXPOSE 80

With these two files, we can now create the Docker image. For this we can use the build command of Docker:

  docker build -t my-docker-website .

The -t flag is used to give the image a name and the dot at the end sets the Build-Context to the current directory.

After the build is finished, we can create a container from the image. For this we use the run command:

  docker run -d -p 8080:80 --name docker-website my-docker-website

The -d flag runs the container in detached mode, -p maps the container port 80 to the port 8080 on the host system, and --name gives the container a name. The last argument is the name of the image to run the container from.

After the container is running, you can access the website by opening a browser and accessing http://localhost:8080. You can check if the container is running by using the docker ps command, which lists all running containers.

  docker ps

The container can be stopped with the stop command and removed with the rm command:

  docker stop docker-website
docker rm docker-website

If the image is no longer needed, it can be removed with the rmi command:

  docker rmi my-docker-website

Docker Alternatives

Next to Docker, there are other containerization tools available. One of the most well-known alternatives is Podman. Podman is an Open-Source container engine, which in contrast to Docker, does not require a daemon to run. This allows Podman to run containers as regular users without the need for root. Podman is also compatible with Docker images and containers, making it a good drop-in replacement for Docker.

Resources

Docker
Docker - Wikipedia
OS-level virtualization - Wikipedia

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Last updated 07 Mar 2025, 08:25 +0100 . history