v3.1.0

The version bump is warranted by a number of fixes for Podman
compatibility, with the biggest improvement being that the user's account
wasn't properly created in the container, so their home
directory was not writeable, which rather defeats the whole point of running DUE.

Additionally, fixes were added for RedHat Docker image compatibility,
in that the version of 'adduser' in Fedora 33 does not support the
same arguments as the version of 'adduser' in Debian.
Sp while DUE would allow running Debian containers on a Red Hat system,
running Red Hat containers would fail on user account creation in the container.

I suspect these sorts of compatibility issues will come up again,
so this may provide a useful template for handling them.

The sha1sum of the attached due_3.1.0-1_all.deb below is
2aa6d18a0dd213f3bf16d0b75bbda7ebf992e441