In the newer betas, the loopback image that unRAID uses for Docker is BTRFS but the underlying storage can be XFS, ReiserFS, or BTRFS.
So in my case... my cache drive is XFS, so when I map the data volume for the splunk container to /mnt/cache/apps/splunk, splunk sees that it is running on an XFS filesystem....
I haven't been able to think of a good workaround for running it on BTRFS that would be docker friendly...