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)
Merge pull request #4540 from unclejack/debootstrap_upgrade_packages

Run dist-upgrade after repo updates in mkimage-debootstrap