Drop database connection information into settings.php file for new codebase

[geerlingguy]

This way the container can be scaled to more than 1 replica in Kubernetes (see geerlingguy/raspberry-pi-dramble#111), and it can also be replaced and still use an existing database.

Basically, the entrypoint script, if it needs to download the Drupal codebase, should also copy default.settings.php over to settings.php and append environment-variable DB connection information, like:

$databases['default']['default'] = [
  'driver' => 'mysql',
  'database' => getenv('DRUPAL_DATABASE_NAME'),
  'username' => getenv('DRUPAL_DATABASE_USERNAME'),
  'password' => getenv('DRUPAL_DATABASE_PASSWORD'),
  'host' => getenv('DRUPAL_DATABASE_HOST'),
  'port' => getenv('DRUPAL_DATABASE_PORT'),
];

I also need to do a site install and see what else is added to the default settings.php—I think it also drops some sort of encryption key in there. That should be set as an env variable as well, if that's the case, with instructions for how to generate it.

[geerlingguy]

Got this pretty much working. So much nicer.