Use container images from git-registry.thm.de

The images from the frag.jetzt-backend are now also on git-registry.thm.de. We can use these images in place of the ones on docker hub: https://git.thm.de/arsnova/frag.jetzt-backend/container_registry/7

Since the frag.jetzt-backend can only be accessed internally and not publicly, an interested party has to log in with Docker at git-registry.thm.de and cannot pull the frag.jetzt-backend image anonymously.

Therefore, we should either continue to use the Docker Hub image or set the project frag.jetzt-backend to the visibility "public". This also makes the image publicly and anonymously accessible (just like the one on docker hub).