Refactor Dockerfile, use pipenv for Python packages
    
    This commit refactors the Dockerfile to use fewer layers when building
    the image. It also re-arranges the order of how things are run to
    benefit some steps like installing Python dependencies.
    
    Ultimately, this commit leverages Pipenv for managing and installing
    dependencies within the project. This is one additional change to
    improving awareness of dependencies and ensuring development,
    production, and staging will always use the same set of dependencies.