Collapse docker/ctafrontend

The directory containing the Dockerfile we use to generate the ctageneric image is called ctafrontend. We should remove the ctafrontend part as this is outdated and not particularly used for anything.

I specifically propose to collapse this structure instead of renaming it, as we likely won't see a use for other directories in docker/. If we want other Dockerfiles, we simply create <name>.Dockerfile as is the typical standard. If we find that we need a lot of these and they are all very complex, then we can decide to add in another directory structure.