Unified command execution in Docker containers.

Also adds a hotfix for a broken test execution based on
an issue with a used library: https://github.com/pytest-dev/pytest/issues/4770



Change-Id: I54aade1eda31cb2584641b95226fbf39d9d298f4
Signed-off-by: peusterm <manuel.peuster@uni-paderborn.de>