Add a Dockerfile and a run script for gunicorn.

I have a habit of running Isso in a container. What better way to run Bussator than alongside in a container, as well?

I've taken the liberty of adding a Dockerfile, taking after Isso's own example, using gunicorn to run a couple of workers. I'm absolutely not at home in Python development and service running, so while I assume this is an OK way of doing things, I'm open to suggestions.

Building the Dockerfile with

docker build -t bussator .

will result in an image bussator:latest, which only needs a directory with a config.ini file mounted to /config:

# Assuming I have a `my_config/config.ini` in the current directory
docker run --rm -p 8080:8080 -v "${PWD}/my_config:/config" bussator:latest

and we're in business.

The amount of testing I've done is limited, but I have a fair amount of confidence in things working. I hope this is of use to the project, I know I'm very enthusiastic about the ability to receive webmentions inline with the comment system!