Support building thirdparty repos

[ShaneMcC]

It would be nice to be able to have makeimg build from 3rd party repos that I do not directly control, but that do have Dockerfiles so that I can have these built and pushed into my registry automatically.

[ShaneMcC]

If this is implemented, probably want some kind of magic so that if multiple people want to build the same repo, you only actually build it once but push it multiple times.