Help identify you are in a Docker container

Add environment variable:

docker=true

This will provide a mechanism for tools like Facter
and Ohai to identify that they are running in a Docker
container.

See https://github.com/puppetlabs/facter/pull/563

See http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/.

Thanks to Paul Nasrat for the pointer to this.