Felix/ci

When looking at the cases-MR I noticed that the ci is overly complicated. So I shortened it. This way it also should need less bandwidth (or rather none) since the docker image is cached after downloaded once in the gitlab runner. And latexmk is just easier to use than guessing that compiling three times might just be the correct number.

Several commits were necessary since the first docker image I found used the outdated debian texlive that gave some error from the keyval package. This one seems to be quite modern.