Add docker volume for PG and Network

[neillindberg]

I had to adjust my docker-compose.yml to get this Quickstart to actually run w/o errors. I found I needed:

1. Network defined and shared in the two container configs.
2. A docker volume created.
3. In the current state of this tutorial if I run docker-compose config I can see errors before the aforementioned changes.

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[craig-osterhout]

Thanks for contributing. The django sample app has been taken out of the docs and moved to the Awesome Compose repo to hopefully facilitate better maintenance. If the issues still exist, please help open an issue there, https://github.com/docker/awesome-compose/blob/master/official-documentation-samples/django/README.md