Upgrade the Docker-based dev environment
    
    The containers now all inherit from a common Fedora 31 image, with only
    Python 3 packages installed. This saves disk space, and creates a
    consistent testing environment.
    
    The base Dockerfile installs Python packages from the distro repository
    instead of PyPi. This creates a stable installation that isn't dependent
    on the latest version of any package, so it should work with less
    adjustment in the future.