code: func Server(index *registrytypes.IndexInfo) (string, error) {
  	if s := os.Getenv("DOCKER_CONTENT_TRUST_SERVER"); s != "" {
  		urlObj, err := url.Parse(s)
  		if err != nil || urlObj.Scheme != "https" {
  			return "", errors.Errorf("valid https URL required for trust server, got %s", s)
  		}
  
  		return s, nil
  	}
  	if index.Official {
  		return NotaryServer, nil
  	}
  	return "https://" + index.Name, nil
  }
docs: // Server returns the base URL for the trust server.