Resolve "Use more elaborate "rules" in GitLab CI/CD configuration"

What does this MR do?

Update the .gitlab-ci.yml configuration to use rules. This allows to fine-tune job execution. In particular, setup and prep stage jobs for preparing the Docker files are now also executed when the respective Dockerfiles change.

Additionally, test:python-tests now has no dependencies and can run immediately.

Is there something that needs to be double checked?

• setup stage triggers when changing docker/dune-env.dockerfile.
• prep stage triggers when changing docker/dune-env-update.dockerfile.
• Triggering works based on changes since last push to branch (not changes in latest commit).

Can this MR be accepted?

• Implemented changes to .gitlab-ci.yml
  • Use rules
  • Run setup and prep stages for source file changes
  • Remove any dependencies of test:python-tests
• [ ] Added/Updated tests
• Added/Updated documentation
• Pipeline passing
• Squash option set
• Delete branch option set
• Added entry to CHANGELOG.md

Assignee: If the Squash option is set, check/update the commit message right before merging!

Related issues

Closes #203 (closed)