Make Tianon the official root "Dockerfile" maintainer, since it's so hard-locked to hack changes most of the time

MAINTAINERS

@@ -3,4 +3,5 @@ Guillaume Charmes <guillaume@dotcloud.com> (@creack)
 Victor Vieux <victor@dotcloud.com> (@vieux)
 Michael Crosby <michael@crosbymichael.com> (@crosbymichael)
 api.go: Victor Vieux <victor@dotcloud.com> (@vieux)
+Dockerfile: Tianon Gravi <admwiggin@gmail.com> (@tianon)
 Vagrantfile: Daniel Mizyrycki <daniel@dotcloud.com> (@mzdaniel)