Correct name of username var, change service name back to postgresql

Author: hartsick

.github/workflows/rails_ci.yml

@@ -16,12 +16,12 @@ jobs:
     env:
       RAILS_ENV: test
 
-      DB_HOST: db
-      DB_PORT: 5432
-      DB_USER: rails
-      DB_PASSWORD: password
+      db_host: postgresql
+      db_port: 5432
+      db_username: rails
+      db_password: password
     services:
-      db:
+      postgresql:
         image: postgres:latest
         ports:
           - "5432:5432"