Add extra info about test database connection (#311)

Author: linkion

src/ruby-rails-postgres/.devcontainer/docker-compose.yml

@@ -40,6 +40,15 @@ services:
       #    username: postgres
       #    password: postgres
       #    database: myapp_development
+      #
+      #  ...
+      #
+      #  test:
+      #    <<: *default
+      #    host: db
+      #    username: postgres
+      #    password: postgres
+      #    database: myapp_test
 
     # Add "forwardPorts": ["5432"] to **devcontainer.json** to forward PostgreSQL locally.
     # (Adding the "ports" property to this file will not forward from a Codespace.)