Update: xdebug is again available for php 7.3.
Due to an issue with opcache in PHP 7.3.0 and 7.3.1 (the latter is the current version offered by our base images), XDebug is currently crashing php-fpm.
I've temporarily disabled XDebug from the generator when on PHP 7.3 until PHP 7.3.2 comes out. You can follow this github issue to track when a fix is available. In the meantime, if you're seeing a 502 error, remove the extension from php-fpm's dockerfile, then rebuild with `docker-compose build php-fpm`.