Add Git support.
Add Git support.

We want to offer the possibility to quickly build applications' Dockerfile
with RUN cd /opt && git clone <repo> && <installation commands>. To minimize
the amount of images adding an extra layer only for Git, we provide it in
our web server base image.

Fixes T376.

nasqueron/docker-nginx-php-fpm,nasqueron/docker-nginx-php-fpm