code: func (i *Image) Cookie(u string) (string, error) {
  	if i.client.Jar == nil {
  		return "", nil
  	}
  
  	baseURL, err := url.Parse(u)
  	if err != nil {
  		return "", fmt.Errorf("Invalid URL: %s", err)
  	}
  
  	cookies := i.client.Jar.Cookies(baseURL)
  	if len(cookies) == 0 {
  		return "", nil
  	}
  
  	return cookies[0].String(), nil
  }
docs: // Cookie returns the string representation of the first
  // cookie stored in stored client's cookie jar.