Copy files/ to novolume/files/ for Docker Hub builder

Summary:
When the build operation starts, all the current directory
is send to Docker as a part of the build context.

The Docker Hub doesn't currently allow to configure the
path to Dockerfile, only a folder. So, when it tries to
build the image, the following error occurs:

Error build: Forbidden path: ../files.

This change copies the files/ folder as novolume/files/
and provides a Makefile to allow to update it each commit
modifying files/ using cd novolume && make update.

Fixes T787.

Test Plan:
For the Docker build part:
docker build -t nasqueron/nginx-php-fpm:novolume .

For the update part:
cd novolume && make update

Reviewers: Kaliiixx

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Maniphest Tasks: T787

Differential Revision: http://devcentral.nasqueron.org/D342