DEV: Use discourse image for postgres in GitHub Actions (#15291)

The discourse base image already contains a postgres installation, so pulling a separate postgres image is a little wasteful. Using the copy of Postgres in the discourse image saves about 20 seconds on every GitHub actions run.

This commit sets up Postgres with a few performance-improving flags, which we were already using for the `rake docker:test` task (used on our internal CI system).

.github/workflows/tests.yml

@@ -21,7 +21,6 @@ jobs:
       DISCOURSE_HOSTNAME: www.example.com
       RUBY_GLOBAL_METHOD_CACHE_SIZE: 131072
       RAILS_ENV: test
-      PGHOST: postgres
       PGUSER: discourse
       PGPASSWORD: discourse
       USES_PARALLEL_DATABASES: ${{ matrix.build_type == 'backend' && matrix.target == 'core' }}
@@ -32,27 +31,10 @@ jobs:
       matrix:
         build_type: [backend, frontend, annotations]
         target: [core, plugins]
-        postgres: ["13"]
         exclude:
           - build_type: annotations
             target: plugins
 
-    services:
-      postgres:
-        image: postgres:${{ matrix.postgres }}
-        ports:
-          - 5432:5432
-        env:
-          POSTGRES_USER: discourse
-          POSTGRES_PASSWORD: discourse
-          POSTGRES_DB: discourse_test
-        options: >-
-          --mount type=tmpfs,destination=/var/lib/postgresql/data
-          --health-cmd pg_isready
-          --health-interval 10s
-          --health-timeout 5s
-          --health-retries 5
-
     steps:
       - uses: actions/checkout@master
         with:
@@ -67,6 +49,12 @@ jobs:
         run: |
           redis-server /etc/redis/redis.conf &
 
+      - name: Start Postgres
+        run: |
+          chown -R postgres /var/run/postgresql
+          sudo -E -u postgres script/start_test_db.rb
+          sudo -u postgres psql -c "CREATE ROLE $PGUSER LOGIN SUPERUSER PASSWORD '$PGPASSWORD';"
+
       - name: Bundler cache
         uses: actions/cache@v2
         with: