Add Drupal example

Something more real-worldish, and more Dockerish, than examples I've seen so far (e.g. no mega-container with Apache+PHP+Composer+everything-under-the-sun inside).

If you're going to use Docker, don't jam in all the bad things about legacy deployment into one massive 1.2+ GB container!

Also, if there's any easy way to work on _simple_ configurability (á la [Drupal VM](https://www.drupalvm.com/) to make something like a 'Drupal VM lite', I'd like to explore it. So far most of the _good_ Docker Drupal examples I've seen are a pain to customize/adapt, while most of the _flexible_ Docker Drupal examples I've seen are slow, bloated, messy, and offer very little benefit over using plain old VMs.