allow to push local docker images by digest

[eunomie]

In the case contentDigest is defined (for example after a build
by docker app) try to push local docker image from the digest.
In that case, the image of the service is not defined in the
bundle.json.

If contentDigest is empty, do the same thing as before, so try to resolve
and if it's not resolvable try to push.

The service name is now displayed in error messages to better understand
what's going on on failures.

Signed-off-by: Yves Brissaud yves.brissaud@docker.com

[thaJeztah]

Question (haven't looked closely, so bear with me); will the image that was pushed be tagged? If not, it may be worth checking with the Docker Hub team; I seem to recall that images that have never been tagged may be garbage-collected after some time.

Haven't looked what the UX for this looks like, but there has been prior discussion in docker about allowing push by digest; in that discussion, the format docker push <image-ID/digest> repository/name:tag was rejected to keep the path open for pushing multiple images (docker push image1 image2 image3).

There was an alternative proposal that was "accepted", but on hold, pending containerd integration; moby/moby#38880 (comment)

That alternative uses a flag (name to be decided on, but currently --local) to specify the local image as "source", thus allowing pushing a single image, and tag it under multiple names, e.g. (from that comment);

docker push --local=<local-image>  \
    myregistry/an-image:v1 \
    myregistry/an-image:v1.0 \
    myregistry/an-image:v1.0.2 \
    myregistry/an-image:latest

[eunomie]

Question (haven't looked closely, so bear with me);

np 😉

will the image that was pushed be tagged?

Yes, images are tagged and then pushed, and then the index will allow to not be garbaged collected as they are referenced.

The main workflow was done in #76. This PR only adds the possibility to do it with a digest as the source and not an image.

https://github.com/docker/cnab-to-oci/blob/87394d4609cce25f8ac2e1ae569eee5887dfd1a9/remotes/fixup.go#L218-L228

[radu-matei]

Manually tested, LGTM.

[silvin-lubecki]

LGTM