Enforcing new image tags to be rebuilt when merging an MR

It's happened a few times now in Mesa that the post-merge CI pipeline got broken due to the pre-merge one using older images which could no longer be rebuilt (due to other changes being merged between when those images were built and when the MR was merged). Latest example: mesa/mesa!5905 (merged)

It would be nice to have some kind of mechanism to enforce in the pipeline before merging an MR that image tags which don't exist in the upstream registry yet are built from scratch. (Only if an existing downstream image is older than the target branch?)