add extra_hosts to docker-compose.yml

add a commented out `extra_hosts` to be uncommented incase of the usage
of `docker-machine` for mapping the `laravel.dev` url to the VM IP
inside the VM container.

docker/docker-compose.yml

@@ -17,6 +17,8 @@ php-nginx:
     - redis
     - beanstalkd
   privileged: true
+  # extra_hosts:
+  #   - "laravel.dev:xxx.xxx.xxx.xxx"
 
 # MySQL Container #----------------------------------------
 mysql: