Fixes #23314 - Add Docker to virtual facts

Adds support to the virtual facts to detect if Facter is running inside a Docker container.

It uses the /proc/1/environ file and checks for the string:

    docker=true

See http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/ for more details of this.

See related PR in Docker - https://github.com/dotcloud/docker/pull/2939