Make container work with composer create-project for Drupal 8.8+

[geerlingguy]

There are a couple more changes I'd like to make after completing #22 — with Drupal 8.8+, the recommended way to begin a new Drupal site codebase is with composer create-project, e.g.:

composer -n create-project drupal/recommended-project:^8.8@dev my-project

This is a bit different than other methods, because:

• It's not a git checkout
• It already has the composer install stuff done
• The 'recommended-project' layout places the web files in a 'web/' subdir (which is how drupal-project template did/does it)

And I'd also like to add a DRUPAL_RUN_COMPOSER_INSTALL env var as part of this, for those who wish to run it after a tarball or git clone.