feat: Add built-in PostgreSQL for simple production setup (#2345)

* feat: Add built-in PostgreSQL for simple production setup

Fixes #2321.

* Use fork of embedded-postgres for cache path

preinstall.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+set -eu
+
+USER="coder"
+
+# Add a Coder user to run as in systemd.
+if ! id -u $USER >/dev/null 2>&1; then
+	useradd \
+		--system \
+		--user-group \
+		--shell /bin/false \
+		$USER
+fi