Remove all app containers before removing volume

If volume location changes, we re-create the volume. However, volume
can only be removed if all the containers using it are deleted. For
example, the scheduler might be running a container using it.

src/scripts/rmvolume.sh

@@ -24,6 +24,8 @@ if [[ "${BOX_ENV}" == "test" ]]; then
     [[ "${volume_dir}" != *"./cloudron_test/"* ]] && exit 1
 fi
 
-rm -rf "${volume_dir}"/*
-# mount points cannot be deleted
+# this removes hidden files
+find "${volume_dir}" -maxdepth 1 -mindepth 1 -exec rm -rf '{}' \;
+# volume could be a mount point that cannot be deleted
 rmdir "${volume_dir}" || true
+