Use official images in rails tutorial

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

Author: bfirsh

docs/rails.md

@@ -28,7 +28,7 @@ Next, we have a bootstrap `Gemfile` which just loads Rails. It'll be overwritten
 Finally, `fig.yml` is where the magic happens. It describes what services our app comprises (a database and a web app), how to get each one's Docker image (the database just runs on a pre-made PostgreSQL image, and the web app is built from the current directory), and the configuration we need to link them together and expose the web app's port.
 
     db:
-      image: orchardup/postgresql
+      image: postgres
       ports:
         - "5432"
     web:
@@ -62,19 +62,18 @@ Now that we've got a new `Gemfile`, we need to build the image again. (This, and
 
     $ fig build
 
-The app is now bootable, but we're not quite there yet. By default, Rails expects a database to be running on `localhost` - we need to point it at the `db` container instead. We also need to change the username and password to align with the defaults set by `orchardup/postgresql`.
+The app is now bootable, but we're not quite there yet. By default, Rails expects a database to be running on `localhost` - we need to point it at the `db` container instead. We also need to change the database and username to align with the defaults set by the `postgres` image.
 
 Open up your newly-generated `database.yml`. Replace its contents with the following:
 
     development: &default
       adapter: postgresql
       encoding: unicode
-      database: myapp_development
+      database: postgres
       pool: 5
-      username: docker
-      password: docker
-      host: <%= ENV.fetch('DB_1_PORT_5432_TCP_ADDR', 'localhost') %>
-      port: <%= ENV.fetch('DB_1_PORT_5432_TCP_PORT', '5432') %>
+      username: postgres
+      password:
+      host: db_1
 
     test:
       <<: *default