docs: @VisibleForTesting
    protected void startExecutor(final int container, Set<PackingPlan.InstancePlan> instances) {
      LOG.info("Starting a new executor for container: " + container);
  
      // create a process with the executor command and topology working directory
      final Process containerExecutor = startExecutorProcess(container, instances);
  
      // associate the process and its container id
      processToContainer.put(containerExecutor, container);
      LOG.info("Started the executor for container: " + container);
  
      // add the container for monitoring
      startExecutorMonitor(container, containerExecutor, instances);
    }