Add volume and version number

docker-compose.yml

@@ -1,3 +1,5 @@
+version: '2.1'
+
 services:
   protonmail-bridge:
     image: shenxn/protonmail-bridge
@@ -8,8 +10,9 @@ services:
     ports:
       - 1025:25/tcp
       - 1143:143/tcp
-    volumes:
-      - protonmail:/root
     restart: unless-stopped
     stdin_open: true 
     tty: true
+volumes:
+  protonmail:
+    name: protonmail