ci_docker

The CI is configured to:

• run test suite and formatters when there is a merge request on "stable" and "develop" and when there is a commit on "stable".
• build docker and pip package when a tag is pushed.

This was tested in the repo: https://gitlab.synchrotron-soleil.fr/PA/collective-effects/python_cicd

I added a dockerfile based on mine but we can converge toward a standard one. How do you want do deal with optional dependencies like pyPIC & pycollef ? We could do several images per release ?