Update `upgrading metabase` section

While it might be obvious to some, I think its a good idea to note.

docs/operations-guide/start.md

@@ -58,6 +58,10 @@ How you upgrade Metabase depends on how you are running it. See below for inform
 #### Docker Image
 If you are running Metabase via docker, then you simply need to kill the Docker process and start a new container with the latest Metabase image. On startup, Metabase will perform any upgrade tasks it needs to perform, and once it's finished you'll be running the new version.
 
+To pull the latest Metabase:
+
+    $ docker pull metabase/metabase:latest
+
 #### Jar file
 If you are running the JVM Jar file directly, then you simply kill the process, replace the .jar file with the newer version and restart the server. On startup, Metabase will perform any upgrade tasks it needs to perform, and once it's finished you'll be running the new version.