Dockerized for easier local development

[adelevie]

[edmorley]

Hi!

In #4 a dev-requirements.txt was added and the README updated with contributing setup steps.

With this, I wonder if a Docker environment is overkill?

For anyone with an existing Python install, they will likely just use the requirements file then run pytest outside of Docker. And for anyone who doesn't have the relevant Python version, they can always precede those steps with eg: docker run --rm -it python:3.6-slim