This header was used for fallbacks to v1 registries, but it's no longer
used, and marked optional / legacy in the OCI distribution-spec:
https://github.com/opencontainers/distribution-spec/blob/v1.0.0/spec.md#legacy-docker-support-http-headers
> Because of the origins this specification, the client MAY encounter
> Docker-specific headers, such as `Docker-Content-Digest`, or
> `Docker-Distribution-API-Version`. These headers are OPTIONAL and
> clients SHOULD NOT depend on them.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
b5835646eb