Ci/add branch to image registry dumux side

What this MR does / why does DuMux need it:

Since we broke the dumux pipeline while trying to build new images for testing, it was decided to add the branch name of dumux-docker-ci into the images we test our code on. (dumux-docker-ci#14) This branch name now needs to be incorporated into the groups where we use the images.

Notes for the reviewer

Expected behavior is that default dumux-docker-ci branch name is master. If someone wants to test on manually build images, it is required to create a branch in dumux-docker-ci and manually trigger the pipeline to push these images to the registry. Here one would need to overwrite a variable to make sure the right image is targeted.

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• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.
• (if not applicable remove) are newly introduced or modified physical values/functions backed up with a scientific reference (including doi) in the docs?
• (if not applicable remove) if the examples are modified, is the documentation regenerated (using generate_example_docs.py)