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).

script/start_test_db.rb

@@ -0,0 +1,30 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+BIN = "/usr/lib/postgresql/#{ENV["PG_MAJOR"]}/bin"
+DATA = "/tmp/test_data/pg"
+
+def run(*args)
+  system(*args, exception: true)
+end
+
+should_exec = false
+while a = ARGV.pop
+  if a == "--exec"
+    should_exec = true
+  else
+    raise "Unknown argument #{a}"
+  end
+end
+
+run "#{BIN}/initdb -D #{DATA}"
+
+run "echo fsync = off >> #{DATA}/postgresql.conf"
+run "echo full_page_writes = off >> #{DATA}/postgresql.conf"
+run "echo shared_buffers = 500MB >> #{DATA}/postgresql.conf"
+
+if should_exec
+  exec "#{BIN}/postmaster -D #{DATA}"
+else
+  run "#{BIN}/pg_ctl -D #{DATA} start"
+end