Mention that we can use prebuilt images in docs. (#313)

* Mention that we can use prebuilt images.

Instead of building our own.

* Mention that we need to comment out `build`.

If we want to use prebuilt.

Running-Mastodon/Docker-Guide.md

@@ -26,10 +26,12 @@ You will need to fill in, at least: `LOCAL_DOMAIN`, `LOCAL_HTTPS`, and the `SMTP
 
 ## Building the app
 
-Before running the first time, you need to build the images:
+If you want to build your own image, run the command below:
 
     docker-compose build
 
+If you want to use prebuilt images on Docker Hub, just comment out all `build` keys in `docker-compose.yml` and continue.
+
 Now the image can be used to generate secrets. Run the command below for each of `PAPERCLIP_SECRET`, `SECRET_KEY_BASE`, and `OTP_SECRET` then copy the results into the `.env.production` file:
 
     docker-compose run --rm web rake secret