add basic packaging with Docker

Add Dockerfile and associated configuration for a basic containerised deployment. The container uses a new settings module, lookupproxy.settings.docker. This uses whitenoise[1] to serve static files, disables debug and allows all hosts. Add a docker entrypoint which collects the static files and then serves the application via gunicorn.

[1] http://whitenoise.evans.io/en/stable/

Required for https://github.com/uisautomation/usvc-ansible/issues/1