Send an email some time before due Date

Hi,
I am trying to send an email to notify the user that their task is due in an hour using the example mentioned here: https://github.com/camunda-consulting/code/tree/master/snippets/task-overdue-job-handler. I am setting the due date in the workflow as well.
I modified some classes in the example above to suit my needs. For example in CreateWatchDuedateJobTaskListener, I changed the reminderDate variable to be
final Date reminderDate = new Date(dueDate.getTime());
In UserTaskOverdueJobHandler.execute method, I have the logic to send the email. I am using the same logic to send the email when the task is assigned to the user and it works just fine. However, I don’t think the execute method in UserTaskOverdueJobHandler is being called and as a result, the user is not being notified of the approaching due date.

I can see the listener and job handler being added in the logs as below:
19-Nov-2018 19:12:28.918 INFO [ContainerBackgroundProcessor[StandardEngine[Catalina]]] org.toronto.ca.bpm.listeners.WatchUserTaskDueDateProcessEnginePlugin.preInit org.toronto.ca.bpm.listeners.WatchUserTaskDueDateProcessEnginePlugin parse listeners added
19-Nov-2018 19:12:30.494 INFO [ContainerBackgroundProcessor[StandardEngine[Catalina]]] org.toronto.ca.bpm.listeners.WatchUserTaskDueDateProcessEnginePlugin.postInit org.toronto.ca.bpm.listeners.WatchUserTaskDueDateProcessEnginePlugin job handler added

Can someone please help me in getting this issue resolved? I am using camunda inside docker from your website.