code: func getTLSConfig(customCACertFile string) (*tls.Config, error) {
  	if len(customCACertFile) == 0 {
  		return nil, nil
  	}
  
  	caCerts := x509.NewCertPool()
  	cert, err := ioutil.ReadFile(customCACertFile)
  	if err != nil {
  		return nil, err
  	}
  
  	if !caCerts.AppendCertsFromPEM(cert) {
  		return nil, fmt.Errorf("credentials: failed to append certificate")
  	}
  
  	return &tls.Config{RootCAs: caCerts}, nil
  }
docs: // getTLSConfig returns a *tls.Config according to whether a user has supplied a customCACertFile. If they have,
  // we return a TLSConfig that uses the custom CA cert as the lone Root CA. If not, we return nil which http.Transport
  // will interpret as the default system defined Root CAs.