Persist metadata backup

SeaweedFS-in-Docker-Swarm.md

@@ -23,7 +23,7 @@ Assuming you have adjusted the Docker stack files and configs below to suit your
 
 ```
 mkdir -p /mnt/{cch,cld,seaweedfs}
-mkdir -p /mnt/seaweedfs/{filer,master,volume}
+mkdir -p /mnt/seaweedfs/{filer,master,metabackup,volume}
 ```
 
 + On a Swarm master node:
@@ -121,6 +121,8 @@ services:
       - public
     environment:
       SHELL_MASTER: seaweedfs_master:9333
+    volumes:
+      - /mnt/seaweedfs/metabackup:/data
     command: 
       - "shell"
       - "lock;"