Two fixes for Docker registry compatibility:
1. Forward the client's Accept header to upstream registries. Docker
clients send specific Accept headers to negotiate manifest format
(Docker v2 vs OCI). Without forwarding, registries default to OCI
format which older Docker daemons reject.
2. Always prefer upstream's Content-Type over the provider's default.
The provider hardcodes manifest types but upstream may return a
different format (e.g. OCI index vs Docker manifest list).
Tested with skopeo against DockerHub, GHCR, and Quay registries.
unkinben
dc34d6669d