This is useful for local development. Any dashboard application
settings that are not set in the parent shell environment will
be given reasonable defaults when appropriate. Having the settings
inside the Django application means that tools that don't always
honour environment settings will still be able to load the
project with a suitably customized local_settings.py.

The README.md has been updated to explain the workflow.

Care has been taken to leave the container image deployment steps
exactly as they were, so as not to introduce any regression. The
"setup-env.sh" script can probably be removed at some point.

See
https://www.agileleaf.com/blog/a-better-way-to-manage-settings-py-in-your-django-projects/
for more motivation on this settings approach.