Re-enable phpdbg in console image

[kierenevans]

The upstream php docker image removed phpdbg from fpm images in docker-library/php#1259 to save space (understandably).

As we build our CLI console image from the php-fpm image to save layers, we can restore it based on suggestions in docker-library/php#1260, i.e. copy it from the php docker CLI image.

Alternatively we can do this in https://github.com/inviqa/harness-base-php as an optional step for projects that need it for faster phpspec code coverage.