NFS into Containers
I have a fairly cheap Synology system managing my spinning rust storage. Block storage over iSCSI didn't perform very well, so I ended up on NFS. NFS works well enough, but unfortunately the support to mount a remote export just isn't there yet. You'll need to mount the NFS share on the host, push the mount into the container as a disk object, and adjust permissions as needed. Here's how I've accomplished this.
NFS Mount
mkdir -p /mnt/share
vim /etc/fstab
ip.or.hostname:/volume1/share /mnt/share nfs4 defaults 0 0
mount /mnt/share
Prep Users & Permissions
Personally, I created system users and groups to handle the permissions of each share. I use the user to grant RW permissions, and the group to grant RO. I map the user and/or group to the user/group inside the container that needs access to the share.
addgroup --system --gid 300 mntShareRO
adduser --system --home /mnt/share --uid 300 --gid 300 mntShareRW
chown mntShareRW:mntShareRO /mnt/share
chmod 750 /mnt/share
chmod g+s /mnt/share
To allow a user to be remapped, you'll need to use /etc/subuid
& /etc/subgid
vim /etc/subuid