image/sourced.go
func (i *sourcedImage) Manifest(ctx context.Context) ([]byte, string, error) {
	return i.manifestBlob, i.manifestMIMEType, nil
}
func (i *sourcedImage) Manifest(ctx context.Context) ([]byte, string, error) {
	return i.manifestBlob, i.manifestMIMEType, nil
}
// Manifest overrides the UnparsedImage.Manifest to always use the fields which we have already fetched.