code: public String forProcessInstance(String alias, long processInstanceId) {
          return registry.getContainerId(alias, new ByProcessInstanceIdContainerLocator(processInstanceId));
      }
docs: Looks up container id for given alias that is associated with process instance
  @param alias container alias
  @param processInstanceId unique process instance id
  @return
  @throws IllegalArgumentException in case there are no containers for given alias