Add APCu extension
Summary:
Our downstreams containers use frequently APCu.
The version 4.0.8, released the 2015-11-20, is considered stable.
We so incorporate the extension directly to our base image.
Test Plan:
Build the image, run a container, check phpinfo().
php -i relevant results are available at P137.
Reviewers: Kaliiixx
Subscribers: Nasqueron Docker deployment squad
Projects: Docker images
Maniphest Tasks: T604
Differential Revision: http://devcentral.nasqueron.org/D97