SCHWED-4612: Ignore warning about unused functions by GCC, while the functions are used by unit tests.

This reverts commit ef7bfdb6.

Didn't work with Sphinx 1.6.7 which needs an extra option at the start.
This needs to be fixed cleanly, or we need to upgrade the Sphinx version
requirements

This likely requires an up-to-date sphinx (tested with current version, 2.3.0)

Make install automatically moves the specified into the default share
directory, so the initially build library can go with the whole build dir

biggest change: make it rely on ost docker-container as base image

So sad to remove our beloved tetris. However, it somehow broke with Python3
and I did not have the nerves to fix it. Any resurection effort is very welcome.

- os.path.walk does not exist in Python3 anymore, use os.walk instead
  (changed interface)
- sphinx-build tried really hard to use Python 2. Import sphinx from
  current interpreter instead and directly call its __main__ function