docker-archive support OCI manifest media type

[davidwtf]

No description provided.

[mtrmac]

Thanks for the PR.

This doesn’t work that way.

Locally, without updating SupportedManifestMIMETypes the generic copy code would convert and not send an OCI manifest, so this PR does not affect typical uses like skopeo copy at all.

More importantly, AFAIK this doesn’t really make a difference, the file format as implemented by Docker (who is, by definition, authoritative) is not flexible to the supported manifests. In fact, the “traditional” docker save format does not have a manifest in this sense at all! It uses a docker-archive-specific top-level manifest format.

Nowadays, IIRC, Docker does support using the OCI format in docker save/docker load, but it does not use the “traditional” format for that purpose; uses the full opencontainers/image-spec archive layout (and, on docker save, creates a file which can be read in both the “traditional” and OCI formats).

Implementing these dual-format archives would be, potentially, interesting, but also much more work than this — e.g. the generic design of containers/image/copy really assumes that every copy uses a single format.

And, contemplating the possibility of a future support of these dual-format archives, we actively don’t want to use the “traditional” format to store OCI, or OCI-like, images, because that would preclude using the new dual-format archive format for OCI images in the future.

[davidwtf]

Yes, you are right.

I directly called the containers/image interface in my program without using skopeo.
But I encountered an error: Unsupported manifest type, need a Docker schema 2 manifest, which led me to mistakenly believe that containers/image does not support converting OCI format images to docker-archive format, hence I submitted this PR.

However, after reading your comment, I verified it with skopeo and studied the differences between my program and skopeo's use of containers/image in detail. Then I discovered that when calling copy.Image, I passed PreserveDigests: true, which caused it not to perform manifests conversion. My mistake.

Thank you for your reply. I will close this PR.