explicitly delete temporary build images

[mhofman]

#283 leveraged the docker registry to store the single build images, pull them for testing, and push them for publishing.

After these steps of the workflow have completed we no longer need these images, and they can be deleted. We currently configured the retention to 1d, but we still risk blowing up our storage usage if we have a lot of builds with changes within that period of time.

The depot CLI does not (yet?) support deletion of build images, but we were told we can use the DeleteImage API to do this. The image name has the shape of registry.depot.dev/${PROJECT_ID}:${BUILD_ID}-${TARGET}.

[mhofman]

Actually looks like image list and deletion was recently added. Since it crudely exposes the API, we'd still have to first list the images, filter them, and delete them by digest.

The DeleteImage API does work by image_tags so it's unclear why the CLI uses digests.