Upgrade to final PHP 5.6.40 version
Summary:
PHP 5.6.40 is the final version of PHP 5.
This change also updates keyservers, as SKS doesn't work anymore.
Reference: https://www.php.net/eol.php
Test Plan: Build image
Reviewers: dereckson
Reviewed By: dereckson
Subscribers: Nasqueron Docker deployment squad
Tags: Docker images
Differential Revision: https://devcentral.nasqueron.org/D2524