docker/docker_image_src.go
func (s *dockerImageSource) ensureManifestIsLoaded(ctx context.Context) error {
	if s.cachedManifest != nil {
		return nil
	}

	reference, err := s.ref.tagOrDigest()
	if err != nil {
		return err
	}

	manblob, mt, err := s.fetchManifest(ctx, reference)
	if err != nil {
		return err
	}
	// We might validate manblob against the Docker-Content-Digest header here to protect against transport errors.
	s.cachedManifest = manblob
	s.cachedManifestMIMEType = mt
	return nil
}
func (s *dockerImageSource) ensureManifestIsLoaded(ctx context.Context) error {
	if s.cachedManifest != nil {
		return nil
	}

	reference, err := s.ref.tagOrDigest()
	if err != nil {
		return err
	}

	manblob, mt, err := s.fetchManifest(ctx, reference)
	if err != nil {
		return err
	}
	// We might validate manblob against the Docker-Content-Digest header here to protect against transport errors.
	s.cachedManifest = manblob
	s.cachedManifestMIMEType = mt
	return nil
}
// ensureManifestIsLoaded sets s.cachedManifest and s.cachedManifestMIMEType
//
// ImageSource implementations are not required or expected to do any caching,
// but because our signatures are “attached” to the manifest digest,
// we need to ensure that the digest of the manifest returned by GetManifest(ctx, nil)
// and used by GetSignatures(ctx, nil) are consistent, otherwise we would get spurious
// signature verification failures when pulling while a tag is being updated.