code: func SimpleDockerRef(p *d2acommon.ParsedDockerURL) string {
  	var sds string
  	if p.IndexURL != defaultIndexURL {
  		sds += p.IndexURL
  	}
  
  	imageName := p.ImageName
  	if p.IndexURL == defaultIndexURL && strings.HasPrefix(p.ImageName, defaultRepoPrefix) {
  		imageName = strings.TrimPrefix(p.ImageName, defaultRepoPrefix)
  	}
  
  	if sds == "" {
  		sds = imageName
  	} else {
  		sds = path.Join(sds, imageName)
  	}
  
  	digest := p.Digest
  	tag := p.Tag
  	if digest != "" {
  		sds += "@" + digest
  	} else {
  		if tag != defaultTag {
  			sds += ":" + tag
  		}
  	}
  
  	return sds
  }
docs: // SimpleDockerRef returns a simplified docker reference. This means removing
  // the index url if it's the default docker registry (registry-1.docker.io),
  // removing the default repo (library) when using the default docker registry