Are there plugins or anything that will allow Dokku to run queues of batch jobs? Skimming the docs there is clear support for Heroku-style apps and one-offs that can be launched via cron, not so much for batch although it would be quite useful.
You may wish to look into a message processing system in your language of choice and run that as a daemon in Dokku. We have plugins for various datastores, and commonly I see folks just connect their worker processes to that. It's much lighter weight than spawning new containers for each workload.