Running UrBackup as a Docker container

I'm trying to install UrBackup https://hub.docker.com/r/uroni/urbackup-server using Portainer

My stack looks like this:

version: '2'

services:
  urbackup:
    image: uroni/urbackup-server:latest
    container_name: urbackup
    restart: unless-stopped
    environment:
      - PUID=1000 # Enter the UID of the user who should own the files here
      - PGID=100  # Enter the GID of the user who should own the files here
      - TZ=America/Argentina/Buenos_Aires
    volumes:
      ## Docker images:  /srv/dev-disk-by-uuid-d361dfc8-2d6b-47e0-82f9-c259cf88b4ab/docker
      ## Docker volumes: /srv/mergerfs/mainpool/dockerdata
      - /srv/mergerfs/mainpool/dockerdata/urbackup/database:/var/urbackup
      - /srv/mergerfs/mainpool/dockerdata/urbackup:/backups
    network_mode: "host"

When I deploy the stack, I get errors from UrBackup about not being able to write to the /backup directory, so I tried the following change in the volume's sections above:

volumes:
- /srv/dev-disk-by-uuid-d361dfc8-2d6b-47e0-82f9-c259cf88b4ab/docker/urbackup/database:/var/urbackup
- /srv/dev-disk-by-uuid-d361dfc8-2d6b-47e0-82f9-c259cf88b4ab/docker/urbackup:/backups

Notice that the /backup docker path is now a absolute path instead of a shared folder on the mergerfs poll

The approach above worked but not sure if it is optimal as the backup files will be stored there and will change over time. Alse, I'd like to add more disks to the pool in the future, adding more storage (now I only have 4TB).

Would it be a permissions problem on the pool? I'm running Docker with 'dockeruser' (PID: 1000, UID: 100)