README additions & build fixes

[DasSkelett]

This splits the first chunk from #87, these changes are mostly unrelated with #87.

Changes

• Some fixes and additions for the README
• Update rules_python in WORKSPACE to fix building with up to date Bazel releases, in particularly this error:

  $ bazelisk build //wgkex/broker:app
  ERROR: ~/.cache/bazel/_bazel_user/0cb50d472ede3feaa890df93c18ed4c0/external/pip/pypi__flask/BUILD:5:11:
  no such package '@pip//pypi__importlib_metadata':
  BUILD file not found in directory 'pypi__importlib_metadata' of external repository @pip.
  Add a BUILD file to a directory to mark it as a package. and referenced by '@pip//pypi__flask:pypi__flask'
  

• Unfortunately, this breaks building with the old Bazel version frmo the Docker image (from 2020).
  The upstream Docker image is dead, so now we build our own from the python base image.

[awlx]

🎉