This adds support to push docker images to quay.io, like other projects in
the iovisor org.

It separates docker image builds into a separate github workflow, and
refactors the package building process slightly, to be generic, in order to
create builds for both ubuntu 16.04 and ubuntu 18.04.

This provides a means to distribute intermediate apt packages between releases,
and also enables uploading these as CI artifacts.

As recent releases have not annotated their tags, it drops the requirement for
tags to be annotated in selecting the version to use.