I periodically go into Docker and EXPORT the container into a subdirectory in my Docker shared folder. And, I use hyperbackup to back up the entire Docker shared folder (which includes all the data, settings, etc for all my docker containers/applications) to another NAS every two hours.
I just exported my containers to the docker shared folder, I never included that. Currently I run snapshots on docker shared folder every 12 hours, and hyper backup ever 24 hours off site.