I think you're writing these as "possible" extension points, but IMHO the likihood of ever extending here is probably quite low.  I'd leave these out and just directly use the stuff from ``dockercmd``.  At some point in the future, if/when there's a real and present need to extend here, than it can be done then.  No need to solve all problems on the first implementation, it's okay to leave some things hard-coded or even undefined completely.