Add CI tests and CD pipeline (#117)
    
    Add Github Actions workflow

    Reduce interactivity in a CI environment

    Change to systemd-nspawn from chroot

    Clean up some magic strings as variables

    Remove firefox (Cisco GPG repo problems)

    Unmount everything and xz resulting image

    

    Co-authored-by: Nikhil Jha <nikhiljha@users.noreply.github.com>