image/manifest: accept OCI media types in Docker schema2 manifests

[lumincui]

What

Accept OCI image config and layer media types when validating Docker schema 2 manifests in go.podman.io/image/v5/manifest.

This allows Schema2FromManifest / SupportedSchema2MediaType to handle Docker schema2 manifests with OCI descriptor media types, including application/vnd.oci.image.layer.v1.tar+gzip.

Why

Some registries return Docker schema2 manifests whose config/layer descriptors use OCI media types. Today those fail before copy/inspection can proceed, for example through skopeo copy --all:

unsupported docker v2s2 media type: "application/vnd.oci.image.layer.v1.tar+gzip"

Fixes #977. This replaces the PR that was first opened against the migrated containers/image repo: containers/image#2973.

Testing

$ docker run --rm -v "$PWD":/src -v ~/.cache/go-mod:/go/pkg/mod -v ~/.cache/go-build:/root/.cache/go-build -w /src/image golang:1.25 sh -lc 'timeout 420 /usr/local/go/bin/go test ./manifest -run TestSupportedSchema2MediaType -count=1 -v'
=== RUN   TestSupportedSchema2MediaType
--- PASS: TestSupportedSchema2MediaType (0.00s)
PASS
ok  	go.podman.io/image/v5/manifest	0.002s

Also verified the same code path in a patched skopeo v1.23.0 build during registry mirroring: manifests with OCI layer media types copied successfully after this change.

[mtrmac]

Per #977:

Images with mixed MIME types are invalid, and have always been invalid. This code has been rejecting them since 2019, and with every passing month the case for accepting them grows weaker.

So the first recommendation must be to fix the producer, and relaxing these checks would, by now, need an exceptionally strong argument that these images are 1) widespread in the ecosystem and 2) unlikely to be fixed.

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https://github.com/podman-container-tools/container-libs/blob/main/CONTRIBUTING.md#sign-your-prs please, we can’t really even look at code submissions with unclear copyright status.