oci/archive/oci_src.go
func (s *ociArchiveImageSource) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
	return s.unpackedSrc.GetSignatures(ctx, instanceDigest)
}
func (s *ociArchiveImageSource) GetSignatures(ctx context.Context, instanceDigest *digest.Digest) ([][]byte, error) {
	return s.unpackedSrc.GetSignatures(ctx, instanceDigest)
}
// GetSignatures returns the image's signatures.  It may use a remote (= slow) service.
// If instanceDigest is not nil, it contains a digest of the specific manifest instance to retrieve signatures for
// (when the primary manifest is a manifest list); this never happens if the primary manifest is not a manifest list
// (e.g. if the source never returns manifest lists).