Add the following xdebug.ini in your docker container:
zend_extension=/usr/lib/php/20170718/xdebug.so
xdebug.remote_enable=1
xdebug.remote_handler=dbgp
xdebug.remote_port=9000
xdebug.remote_autostart=1
xdebug.remote_connect_back=off
xdebug.idekey=debugit
xdebug.remote_host=docker.for.mac.localhost