Extract docker/ in a dedicated project

the docker stack is getting fat, and most of the time, there is no real close dependency/link between swh-environment and what is done in the docker/ directory.

With the idea of making docker tests more self-contained (less dependent on The Internet™), we really do not want to add test datasets (directly or via git-lfs or similar) in the main swh-environment repo.

So it make sense to think about splitting swh-environment in 2 to have the docker/ content managed in a dedicated repo/project.