refactor a bit

• move every docker build material in a per-container dedicated directory
• use entrypoint.sh files as entrypoints
• simplify a lot the Dockerfile: we only pip install what's necessary, the possibility of using swh packages from sources is done by detecting and installing every repo found in /src/swh-*. It's the responsibility of the user to mount local source directories via docker volumes.
• rewrite the docker-compose file accordingly, using env vars when needed.