Add debug options to wp-config.php locally

[pento]

We currently use WP-CLI to add the debug options to wp-config.php, but because it's going via Docker, it's relatively slow for such a simple task. Doing it locally would save a bunch of time during startup.

[pento]

This was partially circumvented in #36, as the debug options are only added to wp-config.php when it's created, rather than every time the server restarts.

It's still slow, though.