code: func (mb *configManifestBuilder) AppendReference(d distribution.Describable) error {
  	descriptor := d.Descriptor()
  
  	if err := descriptor.Digest.Validate(); err != nil {
  		return err
  	}
  
  	mb.descriptors = append(mb.descriptors, descriptor)
  	return nil
  }
docs: // AppendReference adds a reference to the current ManifestBuilder