Allow the nginx-proxy to have an external network configuration in the override config file

This MR move the network configuration from the docker-compose.yml file to the override one in order to allow developers to easily switch between the network_mode: bridge feature or the external named network.

It takes the opportunity to delete the companion configuration which has not worked for many years and to change the mailhog vhost from mailhog.local to mailhog.localhost (to be consistent with new project configuration)