Testing for zfs failed when running docker container with ZFS on Arch Linux

I’m trying to setup UrBackup using docker image (latest: 2.5.30) and ZFS on Arch Linux.
I read a manual and container’s explanation.
Followings are my ZFS environment and some commands to run the container.

[maria@archtst bk]$ sudo zpool list
NAME   SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
bk    19.5G  59.8M  19.4G        -         -     0%     0%  1.00x    ONLINE  -
[maria@archtst bk]$ sudo zfs list
NAME                USED  AVAIL     REFER  MOUNTPOINT
bk                 59.6M  18.8G     58.3M  /bk
bk/backups          296K  18.8G      104K  /bk/backups
bk/backups/files     96K  18.8G       96K  /bk/backups/files
bk/backups/images    96K  18.8G       96K  /bk/backups/images
bk/configs          116K  18.8G      116K  /bk/configs
[maria@archtst bk]$ echo "/backups" | sudo tee /bk/configs/backupfolder
/backups
[maria@archtst bk]$ echo "bk/backups/images" | sudo tee /bk/configs/dataset
bk/backups/images
[maria@archtst bk]$ echo "bk/backups/files" | sudo tee /bk/configs/dataset_file
bk/backups/files
[maria@archtst bk]$ sudo docker run -d \
   --name urbackup \
   --restart unless-stopped \
   -e TZ=Asia/Tokyo \
   -v /bk/backups:/backups \
   -v /bk/database:/var/urbackup \
   -v /bk/configs:/etc/urbackup \
   --network host \
   --device /dev/zfs \
   --dns=192.168.1.4 --dns=192.168.1.6 \
   uroni/urbackup-server:latest
Unable to find image 'uroni/urbackup-server:latest' locally
latest: Pulling from uroni/urbackup-server
32fb02163b6b: Pull complete
862da9dcf0bd: Pull complete
8601bd6db352: Pull complete
Digest: sha256:809a56982d1954d15f8d786cae8081cb998e30d7c0e5b9ce6315dc092928c992
Status: Downloaded newer image for uroni/urbackup-server:latest
809fdaa8f4d83fb1cf7f3994304b32da33e8a6ae3dfcbe0f5c5a70f000d34a6e
[maria@archtst bk]$ sudo docker exec -it urbackup urbackup_snapshot_helper test
Testing for btrfs...
ERROR: not a btrfs filesystem: /backups/testA54hj5luZtlorr494
TEST FAILED: Creating test btrfs subvolume failed
Testing for zfs...
TEST FAILED: Creating test zfs volume "bk/backups/images/testA54hj5luZtlorr494" failed
[maria@archtst bk]$

I couldn’t figure out what’s wrong with my settings.
So, I need help and more info about how to set up… thx.