Take a look at this, maybe it will help you.
RE: OMV on Raspberry and SD Card full!
Do it like this:
- Stop the docker daemon (and all containers with it) systemctl stop docker
- rsync -aHv /var/lib/docker /srv/dev-disk-by-id-usb-WDC_WD10_EARS-00Y5B1_152D00539000-0-0-part2/omv/docker
Copy all old docker contents to new location keeping permissions and possible hard links and be verbose about what it is doing
- change the docker location in the OMV UI
- Restart the docker daemon in the OMV UI (will restart containers as well)
- Check if all your containers are running as expected