When only leaf images are rebuilt, the Packages index might be outdated
and refer to packages that no longer exist in the archive. Always
running "apt-get update" before installing packages avoids this.

Also remove contents of /var/lib/apt/lists as we no longer use it and it
allows to keep the images slightly smaller.

This is the current state of the Git master branch of the DUNE core modules.
The image needs to be rebuilt from time to time.

We use a staged build to make sure to only update the image if the tests for
the core modules pass.