Consider uploading containers to dockerhub

[david-woelfle]

Problem

Build times of containers take aaaaaaaaages.

Potential Solution

Consider uploading build containers to dockerhub. That would furthermore also make results computed which beobench easier to reproduce as one could write something like "This results has been computed using the beobench/sinergym:1.2.3" environment, well at least if you would implement issue #86 to.