code: func (mb *referenceManifestBuilder) References() []distribution.Descriptor {
  	refs := make([]distribution.Descriptor, len(mb.Manifest.FSLayers))
  	for i := range mb.Manifest.FSLayers {
  		layerDigest := mb.Manifest.FSLayers[i].BlobSum
  		history := mb.Manifest.History[i]
  		ref := Reference{layerDigest, 0, history}
  		refs[i] = ref.Descriptor()
  	}
  	return refs
  }
docs: // References returns the current references added to this builder