Haha! Well - I’ve actually not been using it for a while, whilst the macOS client issues get sorted out - but I’d been happy with the server side running inside Docker when I was running it. 
I’ll likely be blowing away my current setup and starting again from scratch, when I get to it.
I’m no Docker expert, but I’ve got the following paths linked to /volume1/docker inside the NAS:
/volume1/docker/urbackup-server/log /var/log
/volume1/docker/urbackup-server/database /var/urbackup
/volume1/docker/urbackup-server/backups /backups
…with my Settings > Backup Storage Path set to /backups.
In the startup log, I see this:
2020-06-18 13:02:57: Image mounting disabled: TEST FAILED: guestmount is missing (libguestfs-tools)
2020-06-18 13:02:57: Backup destination cannot handle subvolumes and snapshots. Snapshots disabled.
(I haven’t yet tried installing libguestfs-tools, as I have no use for image backups at present.)
I don’t know if the BTRFS functionality would get presented to a Docker container - but from what you’ve written above, it looks like btrfs-tools would have to be installed anyway?
Thanks!