[WIP] Add base image from the build UpstreamImage if set

A rough in progress change.
I think we need to be updating the actual user's dockerfile, not doing it this way, because depending on what their dockerfile does, it might be performing operations that are affected by the new version of the upstream image.

And now that we're going to be editing their dockerfile anyway, it would make sense to move this logic (adding the env vars) to the same place.  That way we can get back to just doing a single docker build.

@csrwng agree?