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

Not all strategies have base images.  Some strategies have different meaning for base images.  Eventually docker builds will support multiple base images.  We also will want to be able to pull input from multiple base images to create one output images.  None of those are generic to build.

Trigger information mutates the build parameters to create a config.  When recording that trigger info, the structure should look like what we have in deployments to capture it.  

The two use cases are separate and distinct.  Both should be recorded - the trigger is responsible for correctly populating both.