The Dockerfile provides some flexibility in selecting which version of HeAT should be inside
the Docker image. Also, one can choose whether to install from source or from PyPI.

Some code sections had a mix of spaces and tabs, which have now been
convertd into tabs.

Use pytorch 1.11
Fix problem with CUDA package repo keys

NVidia images come with support for HPC systems desirable for our uses.
They work a little differently internally and required some changes.

The tzdata configuration configures the CET/CEST timezone, which seems
to be required when installing additional packages.

There is an issue with pip caches in the image, which led to the final
cache purge to fail in the PyPI release based build. This is fixed
through a final invocation of true.

… docker readme

for more information, see https://pre-commit.ci

Co-authored-by: Claudia Comito <39374113+ClaudiaComito@users.noreply.github.com>