Just tried this and it works for me.
Note that I too see the following message in my logs:
Aug 02 17:29:34 my.smartserver.space dockerd[20759]: time="2018-08-02T17:29:34Z" level=info msg="loading plugin "io.containerd.snapshotter.v1.btrfs"..." module=containerd type=io.containerd.snapshotter.v1
Aug 02 17:29:34 my.smartserver.space dockerd[20759]: time="2018-08-02T17:29:34Z" level=warning msg="failed to load plugin io.containerd.snapshotter.v1.btrfs" error="path /mnt/volume_sfo2_01/docker/containerd/daemon/io.containerd.snapshotter.v1.btrfs must be a btrfs filesystem to be used with the btrfs snapshotter" module=containerd
Aug 02 17:29:34 my.smartserver.space dockerd[20759]: time="2018-08-02T17:29:34Z" level=warning msg="could not use snapshotter btrfs in metadata plugin" error="path /mnt/volume_sfo2_01/docker/containerd/daemon/io.containerd.snapshotter.v1.btrfs must be a btrfs filesystem to be used with the btrfs snapshotter" module="containerd/io.containerd.metadata.v1.bolt"
But the above error is harmless. It is just saying that for btrfs plugin we need btrfs volume. This can be ignored since Cloudron does not use btrfs plugin.
