I have more or less the same kind of setup.
I use the Rsync module with scheduled tasks.
- on iMac account I created a SSH key
- on OMV I registered the SSH key for each account so OMV can access them
- I have one 'PULL' scheduled task for each account
- I activated daily backup at 01:00 AM and email notifications to be sure the backup is performed
The main advantage is that the machine+account you backup only has to provide SSH access.
And that's all, not backup task to setup on the target, everthing is centralized on OMV.
If your docker container is properly configured for SSH, you should be able to use the rsync 'PULL'
How to setup password-less SSH access: https://phoenixnap.com/kb/setup-passwordless-ssh
Rsync 'PULL' mode: https://forum.openmediavault.org/index.php?thread/24751-rsync-pull-mode