LinuxQuestions.org - Docker volume mount of .img file

I was looking for a simple way to archive a large set of small files and not letting size go crazy. If this works, I would just copy a single file as an archive vs spending hours sync'ing the files to a backup location.

From my research, my issue has something to do with the /dev/loop0 but from what I've seen they all say mount it first, then use it. Several try to create the loop in the container, but then you have to create the /dev entries. I'd rather leave that out of the container and let the host deal with it.

I'm trying to map a volume in my docker-compose.yaml, but it never appears in the container...

On the Host

Code:

truncate -s 5G /srv/myimage.img  

mkfs.btrfs /srv/myimage.img 

mount -o loop,discard,compress=lzo /srv/myimage.img /mnt/myimage/

The container (docker-compose snippet):
(I'm using version: '2' but I can use any version if needed.)

Code:

  app:

    build: dockerfolder

    container_name: myContainer

    network_mode: bridge

    restart: unless-stopped

    privileged: true

    device_cgroup_rules:

      - b 7:* rmw

    volumes:

      - /dev:/dev

      - /mnt/myimage:/data

I run this:

Code:

docker-compose exec app bash

df -h

but my volume doesn't appear. If I attempt to use it, it dumps the data to a folder on /mnt/myimage (of the host drive - you have to unmount the img to see it)