upgrade packages after debootstrap

This makes mkimage-debootstrap upgrade packages after retrieving
updated lists of packages.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)

contrib/mkimage-debootstrap.sh

@@ -219,6 +219,7 @@ if [ -z "$strictDebootstrap" ]; then
 	
 	# make sure our packages lists are as up to date as we can get them
 	sudo chroot . apt-get update
+	sudo chroot . apt-get dist-upgrade -y
 fi
 
 if [ "$justTar" ]; then