use castellum image to run tests

By using the castellum image to run tests, we already have most dependencies.

This makes the setup much faster. However, running the actual tests is for some reason much slower (~9 minutes instead of ~3 minutes). I wonder whether this means that the castellum image itself is somehow slow, which would be bad.

@krause @muecke do you have any idea why this is so slow? Could it be something about a different libc in alpine or something?