* Docker caching for devel
Use more caching features of Docker for building the devel image.
Downsite of this is that for users who heavily build the image it will
eat up more disk space then before.
But I think think only developers are affected here and all others will
pull from the registry.
A regular docker cleanup on the machines will solve this problem.
* Upgrade python to python3
The main Dockerfile will build now a python3 based image.
A copy of the old python2 file is still available for further testing.
Florian Pelgrim