Allow PR'd branches to build their own product base images for test builds

[ianpittwood]

This is a follow-up from #571 where we enabled "-dev" suffixed builds to build from the dev branch for building and testing non-main branches.

Currently, if a non-main/non-dev branch makes breaking changes or has dependent changes in the product base image, the branch will inevitably fail to build as the product base images used will be from the last build by the dev branch. We should be able to devise a tagging or naming strategy to allow non-main/non-dev branches to build and use newly built base images in downstream product images.