Add docker support

This merge request add a very simple and efficient way to run Vtiger with Docker.

It also include the required PHP extension (running on PHP-FPM 7) and config (.user.ini) with Nginx and a MySQL 5.7 docker container.

It also "map" the volume "vtigercrm" has a volume directly inside docker so that developer can still use their prefer IDE and test directly the changes on their localhost on port 7280.

This merge request also add a USAGE.md documentation file to instruct the user on how to install / configure / use Vtiger with Docker.

There is also a Makefile included which is used as a registry for helper command instant of having multiple bash script... In the future developer can add more commands that are related to common task used by all developers.