Add Xdebug

[alikarim]

This is a small change to activate Xdebug via the docker build command.

This is the build command:
docker build -t moodlehq/moodle-php-apache:7.3 --build-arg XDEBUG=1 .
or
docker build -t moodlehq/moodle-php-apache:7.3-xdebug --build-arg XDEBUG=1 .

We can override the arguments:
docker build -t moodlehq/moodle-php-apache:7.3 --build-arg XDEBUG=1 --build-arg XDEBUG_REMOTE_HOST=0.0.0.0 --build-arg XDEBUG_VERSION=2.8.0 .

Thank you for your comments.
Ali

[andrewnicols]

This will be better covered by #173 which proposes always installing xdebug and xhprof, but only enabling them when a runtime flag is passed.

We really don't want to be getting into a situation where we're having build time flags to add features. It helps no-one and possibly even leads to stale cache layer issues.