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

Dockerfile

@@ -95,5 +95,8 @@ RUN cd /go/src/github.com/mattermost/platform/web/react; npm install
 RUN chmod +x /go/src/github.com/mattermost/platform/docker/docker-entry.sh
 ENTRYPOINT /go/src/github.com/mattermost/platform/docker/docker-entry.sh
 
+# Create default storage directory
+RUN mkdir /mattermost/
+
 # Ports
 EXPOSE 80

config/config_docker.json

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