Need to mention that docker image reference in taskcluster.yml needs to be updated

Author: No-Jun Park

android/docker_guide.md

@@ -39,6 +39,8 @@ docker push mozillamobile/focus-android.
 
 Running this command requires that you have an account, are added to the "mozillamobile" organization, and have logged in via the command line: `docker login`. Ask :sebastian if you need to be added.
 
+Once the docker image is pushed, make sure your `taskcluster.yml` is referencing the new docker image with the correct tag number under `payload`.
+
 Note: If there is a failure while building the docker image, this often comes from lack of memory.  Try increasing the memory and swap space on GUI and/or `-m 5g` build argument. 
 
 ## Debugging errors