Since mw/core dev images have been built by blubber, they lost root access, resulting in the following errors when trying to enable xdebug.
ln: failed to create symbolic link '/etc/php/7.2/cli/conf.d/20-xdebug.ini': Permission denied rm: cannot remove '/var/lib/php/modules/7.2/cli/disabled_by_admin/xdebug': Permission denied