code: func (mb *configManifestBuilder) emptyTar(ctx context.Context) (digest.Digest, error) {
  	if mb.emptyTarDigest != "" {
  		// Already put an empty tar
  		return mb.emptyTarDigest, nil
  	}
  
  	descriptor, err := mb.bs.Stat(ctx, digestSHA256GzippedEmptyTar)
  	switch err {
  	case nil:
  		mb.emptyTarDigest = descriptor.Digest
  		return descriptor.Digest, nil
  	case distribution.ErrBlobUnknown:
  		// nop
  	default:
  		return "", err
  	}
  
  	// Add gzipped empty tar to the blob store
  	descriptor, err = mb.bs.Put(ctx, "", gzippedEmptyTar)
  	if err != nil {
  		return "", err
  	}
  
  	mb.emptyTarDigest = descriptor.Digest
  
  	return descriptor.Digest, nil
  }
docs: // emptyTar pushes a compressed empty tar to the blob store if one doesn't
  // already exist, and returns its blobsum.