The btrfs file system bit is fairly easy. Open Storage Manager and see what it says.
Here's my DS218+ you'll see it says "File system ext4": ext4 isn't supported by Virtual Machine Manager but btrfs is.
My other NAS is using btrfs and I can (and do) run VMM on it.
If you can't get the below Docker to work then you may be able to run VMM package and install a Linux virtual machine and run Jacktrip in it.
Docker doesn't care about ext4/btrfs and your DS218+ can install and run the Docker package. There is one Jacktrip container image you might try.
How it works is not something I can help with.