[WIP][DO_NOT_MERGE] add an upstream image field to builds

I don't agree.  UpstreamImage represents information about the new image who's creation triggered this build.  Whether the particular build strategy explicitly consumes that image or not is irrelevant.

furthermore, putting this into the strategy wouldn't address the issue you seem to be raising since some strategies still would have no upstream image, so it's no more appropriate there.  And putting it on each specific strategy that has an explicit upstream image would significantly complicate the code for no real benefit to end users. (Aside from my primary point that this is information about the trigger that initiated this build).