The right approach is to use a named volume backed by a nfs remote share.
I run it on DMS7.2 using a configuration like this:
The NFS Fileservice is configured to use NFSv4.1 (on DSM6.2 NFSv4 should be fine as well),
The NFS permissions on the syno share are like this:
Hostname or IP: 192.168.2.0/24 (cidr of the subnet where the docker hosts are)
Privilege: Read/Write
Squash: No mapping
Security: sys
Allow users to access mounted subfolders
This should allow creating a volume that is able to use the remote share
docker volume create --driver local
–opt type=nfs
–opt o=addr=192.168.2.250,nfsvers=4
–opt device=:/volume1/nfsshare
nfsshare
Note: you need to make sure the uid/gid of the share owner (more preferably a subfolder of the share) is aligned with the uid/gid that executes the main application inside a container.