imagetools create should return the digest of the new image

[Pesa]

Description

I'm using docker buildx imagetools create to create a multi-platform image from previously built single-platform images.

I'd like this command to somehow output the digest of the newly created image, so that subsequent steps and downstream jobs can "atomically" operate (run some checks, inspect, sign, etc.) on that same version of the image without relying on tags, which can be moved.

docker/build-push-action already has a digest output, so I guess I'm asking for a similar feature in imagetools create.

[tonistiigi]

@crazy-max Should we add --metadata-file to imagetools create or do you have better ideas for UX. We can also do it for --dry-run that atm. prints the manifest bytes. Digest is just checksum of the same bytes.