docker: enable containerd snapshotter with backwards-compat

[paralin]

https://docs.docker.com/storage/containerd/

Enabling the new containerd snapshotter:

/etc/docker/daemon.json:

{
  "features": {
    "containerd-snapshotter": true
  }
}

We should enable this only if there are zero existing docker images stored on the system.

When switching snapshotters, the images from the older docker image store will not be visible anymore.

Solution:

• check if there are any legacy images stored in the docker storage dir /mnt/persist/skiff/docker
• check if there is any setting for the snapshotter in /etc/docker/daemon.json already
• if not, add containerd-snapshotter to /etc/docker/daemon.json