Simplify image name
    
    Currently the images are named as "f<version>/fedora-toolbox". This is
    troublesome for new users of toolbox (even those with some background to
    containers) because everywhere the image is advertised or talked about
    as "fedora-toolbox". This is taken care of by Toolbox CLI but has no
    effect on Podman itself (or any other tool capable of working with OCI
    images).
    
    Another pain point is in the Fedora registry[0] all "fedora-toolbox"
    images get a different entry for every version of Fedora. There is no
    single place for all "fedora-toolbox" images.
    
    With this change I propose to only use "fedora-toolbox" as the name of
    the container and make use of VERSION to distinguish between versions of
    Fedora. Currently when you go to the Fedora registry and find an entry
    for "fedora-toolbox" you'll see all previous images. I believe that with
    this change that "feature" will be lost. But I personally find that
    "feature" to be rather confusing because what usually a user wants the
    latest version of a container (I partially base this statement on the
    fact that most images are versioned this way; e.g. Ubuntu on Docker
    Hub[1]).
    
    [0] https://registry.fedoraproject.org/
    [1] https://hub.docker.com/_/ubuntu