@VisibleForTesting
  ImmutableMultimap<Class<? extends JibEvent>, Handler<? extends JibEvent>> getHandlers() {
    return ImmutableMultimap.copyOf(handlers);
  }
Gets the handlers added.

@return the map from {@link JibEvent} type to a list of {@link Handler}s
jib-core/src/main/java/com/google/cloud/tools/jib/event/EventHandlers.java