Fix broken /dev/fd in mkimage-arch.sh

Exactly the same problem as dotcloud/docker#2356

Docker-DCO-1.1-Signed-off-by: Lance Chen <cyen0312@gmail.com> (github: Lance0312)

contrib/mkimage-arch.sh

@@ -57,6 +57,7 @@ mknod -m 666 $DEV/tty0 c 4 0
 mknod -m 666 $DEV/full c 1 7
 mknod -m 600 $DEV/initctl p
 mknod -m 666 $DEV/ptmx c 5 2
+ln -sf /proc/self/fd $DEV/fd
 
 tar --numeric-owner -C $ROOTFS -c . | docker import - archlinux
 docker run -i -t archlinux echo Success.