func (s *ostreeImageSource) GetManifest(ctx context.Context, instanceDigest *digest.Digest) ([]byte, string, error) {
	if instanceDigest != nil {
		return nil, "", errors.Errorf(`Manifest lists are not supported by "ostree:"`)
	}
	if s.repo == nil {
		repo, err := openRepo(s.ref.repo)
		if err != nil {
			return nil, "", err
		}
		s.repo = repo
	}

	b := fmt.Sprintf("ociimage/%s", s.ref.branchName)
	found, out, err := readMetadata(s.repo, b, "docker.manifest")
	if err != nil {
		return nil, "", err
	}
	if !found {
		return nil, "", errors.New("manifest not found")
	}
	m := []byte(out)
	return m, manifest.GuessMIMEType(m), nil
}
func (s *ostreeImageSource) GetManifest(ctx context.Context, instanceDigest *digest.Digest) ([]byte, string, error) {
	if instanceDigest != nil {
		return nil, "", errors.Errorf(`Manifest lists are not supported by "ostree:"`)
	}
	if s.repo == nil {
		repo, err := openRepo(s.ref.repo)
		if err != nil {
			return nil, "", err
		}
		s.repo = repo
	}

	b := fmt.Sprintf("ociimage/%s", s.ref.branchName)
	found, out, err := readMetadata(s.repo, b, "docker.manifest")
	if err != nil {
		return nil, "", err
	}
	if !found {
		return nil, "", errors.New("manifest not found")
	}
	m := []byte(out)
	return m, manifest.GuessMIMEType(m), nil
}
// GetManifest returns the image's manifest along with its MIME type (which may be empty when it can't be determined but the manifest is available).
// It may use a remote (= slow) service.