Upgrade image to PHP 7.2

Summary:
Currently, the image still runs on PHP 5.6,
as previously Phabricator didn't support PHP 7.

In 2018, the last blockers for PHP 7 adoption
by Phabricator was addressed. DevCentral installation
tested Phabricator + PHP 7.1 then 7.2 with success.
It's stable and gives good performance.

As such, this change switches the image to PHP 7
too. That mainly offers three benefits:

• it's a supported PHP version
• it allows to connect to MySQL 8 and faster
• it's modern and faster

The change is transparent and doesn't require
any configuration change.

Test Plan: Tested with https://devcentral.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Differential Revision: https://devcentral.nasqueron.org/D1922