Introduce docker containers into CI

Release notes

Docker images are made available for tagged releases (like before), the latest master commit and the latest commits of feature branches. For each case, there is an AnalysisBase-based image and a ROOT standalone image. More details in README.

The structure of the images changed. They are set up automatically (run them with --help to see options). The CAFCore source code can be found in the directory /CAFCore.

Details

This MR also updates the README and removes rc instructions from it.