draft: ci: uses our own docker image for python
This one is an experimentation based on https://forge.extranet.logilab.fr/cubicweb/dockerfiles to provide custom docker images for the CI to reduce the CI load and the jobs startup time.
This one for example pre-install tox.
The work on https://forge.extranet.logilab.fr/cubicweb/dockerfiles needs to be finished first before mergin this MR.