code: function validateAndCbBackend (instance, urlPort, cb) {
    var backendUrl = instance.dockerUrlForPort(urlPort);
    var err = validateContainer(instance.attrs.container);
    if (err) {
      return cb(err); }
    if (!backendUrl) {
      err = Boom.create(400, 'port not exposed', urlPort);
      return cb(err);
    }
  
    cb(null, backendUrl);
  }
docs: private function that gets the backend url of an instance and verifies the port is exposed
  @param  {Object}    instance  instance model
  @param  {Function}  cb        callback