Update Dockerfile to use Python 3.9

[DisSupEng]

Changes

• Updated Dockerfile to use python:3.9 image
• Added docker-compose.yml file to easily set up development environment
• Updated REAME with my contribution

Addresses issue #333

Note: when running flake8 I get a lot of formatting errors but I haven't changed the code so I'm not sure if this is a pre-existing issue

[DisSupEng]

Hi @JessicaTegner thank you for running those tests. I can see that there is an error for cryptography when using pypy7.3.9.

This is the commit the exception was added in: https://github.com/pyca/cryptography/commit/60d3f709eb96ba5df9bdfede5c024c1f2db9f0e5

It looks to have been added three months ago. I could try pinning cryptography to a previous version but I'm assuming they don't want to use < 7.3.10 for a reason so that might not be the best idea. You could try updating to Pypy 7.3.10?

[JessicaTegner]

@DisSupEng I would say to pin cryptography only on that specific pypy version, as that's the only one failing. I don't know if this is possible though