Container execution w/ SARUS

[ChristianKniep]

A couple of ideas around the container execution. Starting with SARUS as that is what I am after.

Adjustments

• Honor the configuration of an images when it comes to ENTRYPOINT and CMD.
• Use the appropriate runtime options to change the workdir and not to cd self.workdir

Quick fixes

• allow to skip_pull an image pull (that might not be something to consider for production, as this kinda violates the reproducibility)

[ChristianKniep]

Finally fixed the unittests... :)