You need to move your docker folder from the default location at
/srv/dev-disk-by-<id of your storage disk> /docker
Then you'd need to go into OMV-Extras->Docker->Docker Storage and enter the new path. If you want to save your old docker contents, you're going to need to ssh into your system and copy it to your storage drive using the bash shell, something like this
I just had the same problem, the above is what I did to fix it and it works fine.
Fantastic. Thanks for the help.