Add "DEBIAN_FRONTEND" to FAQ

Added some explanation on using ‘DEBIAN_FRONTEND’ to the FAQs. As
discussed here;
https://github.com/docker/docker/pull/7035#issuecomment-50967867

Docker-DCO-1.1-Signed-off-by: Sebastiaan van Stijn <github@gone.nl> (github: thaJeztah)
Comma needed after "this".