proof-of-concept docker layer caching

This is a MR against !548 (merged). Its "docker-image" CI run is much faster b/c it uses cached layers (all the dependencies basically) from the CIwork2 branch (which pretending to be master in this little roleplay).

There is a trivial change here (adding a license to a file without one) to show that only the pip install . part of the Dockerfile happens.

It works: 10 minutes cost for full build is reduced to about 2.5 mins. Much of that time is spend just pulling the previous docker build (which is the minimum that must happen to accomplish this cache usage).