Add warning when using privileged mode.

Docker-DCO-1.1-Signed-off-by: Mrunal Patel <mrunalp@gmail.com> (github: mrunalp)
@vieux this is supposed to make it so that this doesn't break scripts that try to parse the output of programs run within docker.  However it's done wrong, the check should be for whether the `-t` flag was set and not `-i`.