code: func SetExternalKey(controllerID string, containerID string, key string, execRoot string) error {
  	keyData := setKeyData{
  		ContainerID: containerID,
  		Key:         key}
  
  	uds := filepath.Join(execRoot, execSubdir, controllerID+".sock")
  	c, err := net.Dial("unix", uds)
  	if err != nil {
  		return err
  	}
  	defer c.Close()
  
  	if err = sendKey(c, keyData); err != nil {
  		return fmt.Errorf("sendKey failed with : %v", err)
  	}
  	return processReturn(c)
  }
docs: // SetExternalKey provides a convenient way to set an External key to a sandbox