The current OCI image spec implementation in distribution is short the following items in it's OCI image index support:

• need to add support for an optional annotations field to the manifestlist.ManifestList struct (index) we support annotations for each of the manifest descriptors in the list but not on the list itself.
• need to add support for empty platform structs (move to Platform *Platform json:"platform,omitempty" - from current Platform PlatformSpec json:"platform") in manifestlist.ManifestDescriptor.

Choices:

1. Make appropriate changes to docker/distribution/manifest/manifestlist.ManifestList and .ManifestDescriptor and associated serialization APIs in manifestlist.go.
2. Split the OCI index and docker manifest list implementations, requires refactoring of manifestlist use throughout code.

Thoughts? @dmcgowan @stevvooe