Add automatic install in entrypoint if DRUPAL_INSTALL_IF_NOT_INSTALLED true

I never really completed https://github.com/geerlingguy/drupal-container/issues/1 ... I want to make it so, if there are database credentials provided, and an env variable like `DRUPAL_INSTALL_IF_NOT_PRESENT` is `true`, then we use Drush to install Drupal... maybe?

Basically, it would take another step out of the first-use/testing order of operations for this image. Mostly, I'm thinking of how it relates to the Raspberry Pi Dramble. Right now after setting everything up, you have to run through the Drupal installer via UI to complete the Drupal installation.

Not that that's horrible or anything, it just takes a little extra time, and isn't _automatic_. I hate things that aren't _automatic_.

If Drupal is already installed (or `DRUPAL_INSTALL_IF_NOT_INSTALLED` is set to `false`), then this new code that does the installation should not be run.