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Docker

Introduction about Robot Framework in Docker at https://docs.robotframework.org/docs/using_rf_in_ci_systems/docker

Docker is basic tool in software engineering today. However, there are concerns about the attempts of Docker Inc. building some sort of business plan on top of it. Another concern is that docker requires root privileges on the host system. A more and more popular alternative in podman. Podman does not require root privileges and it shares almost the same commands as docker. Thus, you can often use docker as an alias for podman. For instance, to build an image with podman, the command is podman build just like docker build.

However, podman is originally developed by Redhat, so you still might be save from creative business plans after all. Podman also provides a desktop ui for all major operating systems. In contrast to Docker Desktop, Podman Desktop is licensed under Apache 2.0 license and is thus free to use.

Container or Image?

In the beginning the terms container and image maybe be confusing as they seem to refer to the same thing. And image is what you build. container is a running instance of the image. You can compare with classes and objects in Java. A class is the specification, while an object is the running instance. You can start several instances of the same class in Java as you can run several containers of the same image in Docker.

  • docker images command shows the images that are cached on your local environment
  • docker ps command shows containers that are stored on your local environment (inactive containers with -all switch)
  • docker stats command shows live stats the currently running container instances

Build Image

Conluding the previous paragraph, we build a docker image. Robot Framework describe the basics in our context very well: https://docs.robotframework.org/docs/using_rf_in_ci_systems/docker#creating-a-robot-framework-dockerimage

A few teeaks shall be mentioned:

  • Do not split CLI calls that belong together over several RUN statements: Each RUN command will result in a cached layer. If docker decides that a cached layer is good enough, it will not execute the RUN line in a later docker build, which leads to weird effects when an image is build that does not look like the Dockerfile. See the RUN command extenstion of the Browser image in this project as example to concatenate commands that belong together.
  • Create a .dockerignore file for controlling what files you do not want in your image: The hidden .git folder is a classic that you do not want to be in your image. If the .git folder is copied unintentionally with COPY . . to your image, you accidentally ship your complete git repository with all its history together with your image.
  • Take image size in to account:

Run Container

docker run starts a container based on an image. docs.robotframework.org describe a few examples for this.

Once a container is started, you sometimes may want to access the actual container. For instance to analyse what tools are installed what files are actually stored in the image. You can access the command line of a container by running:

docker exec -it <container id> bash