Add argparse to simple app main.py

[c-w]

This prevents errors where unexpected arguments are passed to the script, e.g. due to an incorrect command like docker run my_image -p 5000:5000 which would pass -p 5000:5000 to the main.py script instead of to the docker run command.

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[c-w]

Closing in favor of https://github.com/Microsoft/agogosml/pull/295.