[trixie] debos: Add support for variants and build networkless image

    debos: Add support for variants and build networkless image
    
    There are devices that lack of network interfaces support, due to
    hardware limitations, kernel support or testing setup.
    
    In those cases, the current health check image fails as the
    systemd-networkd-wait-online keeps waiting forever.
    
    By the time of this commit, the tests with the above scenario would rely
    on the (deprecated) singularity images [0]. As these are no longer
    built, let's add support for variant images, e.g. networless.