WIP: Modifies CI to use Docker images for the Python environments

I wrote a new .gitlab-ci.yml that contains a build stage where the Docker images are generated for each environment and only rebuilds the images when the Dockerfile changes, and a test stage that runs the tests using these images.

I don't have a solution yet for the problem where a new Dockerfile is made and new test images are generated without replacing the stable ones. Any help here is very appreciated.