Docker files to version tags

Question was if we want to hook up the automatic creation of docker files to the PROPTI version tags. For now @rweisse proposed to use specific changes (e.g. docker files or python files changed) to the master branch to trigger building new docker files.

We ( @lu-kas ) should think about this. I believe it makes sense to keep the master branch clean and use version tags such that the docker files are using the (hopefully) stable PROPTI version.

This is related to PR #113