create default storage directory in Dockerfile, turn on server storage by default for docker config

config/config_docker.json

@@ -20,8 +20,8 @@
         "PublicLinkSalt": "TO3pTyXIZzwHiwyZgGql7lM7DG3zeId4",
         "ResetSalt": "IPxFzSfnDFsNsRafZxz8NaYqFKhf9y2t",
         "AnalyticsUrl": "",
-        "UseLocalStorage": false,
-        "StorageDirectory": ""
+        "UseLocalStorage": true,
+        "StorageDirectory": "/mattermost/"
     },
     "SqlSettings": {
         "DriverName": "mysql",