Attempt to rationalize a bit workers config in docker

• use a common workers.env file for all worker services,
• move services/swh-loaders-worker/entrypoint.sh in services/swh-worker/ and use where possible in docker-compose.yml,
• merge docker/env/statsd.env in common_python.env.

It could be possible to rationalize more, eg. merging all worker entrypoint.sh
in a single one.