Create docker-compose utils

There is repeatability in the array appending in `Site_WP_Docker.php`. To reduce this as well and make it re-usable for packages, we can possibly create a new site-docker-compose-utils. Create a function for say postfix and assign EE\SiteUtils\get_postfix_array() and append it to the main array going for mustaching.

Similar to postfix, this can be done for the common services like nginx, mailhog, php, db etc and create the utils to get there configs in an array so people can use them as defaults easily. This change would be quite helpful here as well as in current site-command.