docker build's --output flag accepts several output "types" which we don't currently implement: we currently implement "local" and "tar" (without the "platform-split" option), but not "oci"/"docker" or "image"/"registry".

We'll need to implement an "image" exporter which, given either a minimally-viable image record created using RUN-4246, or a proper one committed or pulled using existing logic, will produce a working source reference which the image library can use to write a proper image.

The API which RUN-4246 intends to introduce will not be exported, so this will have to be tested by new unit tests.