Support cross-repository blob mounts if copying within the same registry

[whatsacomputertho]

Overview

The Distribution spec includes mention of the use of cross-repository blob mounts

• https://distribution.github.io/distribution/spec/api/#cross-repository-blob-mount

This is useful if the user is copying a container image between two paths ("repositories") in the same registry. If we know the source image location, then we can just tell the registry to "mount" the blobs at the source path into the destination path rather than uploading each blob individually.

Acceptance

• If a user copies a container image between two paths in the same registry, cross-repository blob mounts are used to speed up the copy process