The OCI distribution spec allows implementations to support deleting manifests
by tag, but also permits returning the `UNSUPPORTED` error code for such
requests. docker/distribution has never supported deleting manifests by tag, but
previously returned `DIGEST_INVALID`.
The `Tag` and `Digest` fields of the `manifestHandler` are already correctly
populated based on which kind of reference was given in the request URL. Return
`UNSUPPORTED` if the `Tag` field is populated.
Signed-off-by: Adam Wolfe Gordon <awg@digitalocean.com>
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