Previously, this image added a new site vhost for Phabricator, and
removed the default vhost provided by nasqueron/nginx-php-fpm.
Instead, we overwrite the default with our own configuration.
This allows to avoid the two files to be present in the same image,
as Docker 1.3 to 1.8 doesn't seem to honour a rm command on an upper
layer than the actual file.
Fixes T546.