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

As weird as it sounds but the proper var. to set is PNG_PNG_INCLUDE_DIR instead
of PNG_INCLUDE_DIR. The latter is overwritten by FindPNG and actually obsolete
(see https://github.com/Kitware/CMake/blob/master/Modules/FindPNG.cmake).

Several chain mappings are valid in this case. Only check whether we found an
automated mapping with strict (stuff must really match) chain_mapping_scheme