Missing WORDPRESS_DEBUG environment variable reference

[rossjcooper]

The documentation misses out the WORDPRESS_DEBUG environment variable which I found to be quite important when using it to develop locally when WP_DEBUG is required to be true.

The only reference to it I have found is in a PR from a couple of years ago.

[tianon]

For reference, the PR in question is docker-library/wordpress#108. 👍

(... and yes, it's an oversight that this isn't documented, and we should include a short blurb about it 👍 ❤️)