Add ability to resolve image digests without pulling entire images #5818

PR #4333 added --resolve-image-digests to docker-compose config, which resolves image tags to their digest to form stable references. However, this relies on the images being available on the Docker host.

It would be useful if Docker Compose could fetch image digests without having to pull the image if the image doesn't exist locally. In CI jobs using Docker-in-Docker, pulling images just for tag digests can be a significant waste of time and network resources.

Digests can be obtained using HEAD requests against /v2/<name>/manifests/<reference> in the v2 API.

https://stackoverflow.com/questions/35186693/what-is-the-docker-registry-v2-api-endpoint-to-get-the-digest-for-an-image#35420411