Add Dockerfile

Hi,

I've created a Dockerfile for sabznbd and thought I propose to include that. After merging, you could add this repo as a automated build on hub.docker.com and people could just do a `docker run -p 127.0.0.1:8080:8080 sabnzbd/sabnzbd` to deploy it. For now, you can use my fork:

    docker run -p 127.0.0.1:8080:8080 fish/sabnzbd

Usage:

        docker build -t sabnzb .
        docker run -p 127.0.0.1:8080:8080 sabnzb