Adding Docker support

[Kuklin István]

I have created a Docker image based on your work sometime before and I am curious if it could be merged or linked to your repository to centralise our efforts as one of your contributors suggested:
kuklinistvan/docker-anki-sync-server#2

Also, I'd like to say thank you to you for maintaining ankisyncd as well as to all the contributors!

[tsudoko]

I put a link in the README. You seem to be much better at maintaining the image than I would be (I don't know anything about Docker at all), so I think it's a good solution.

[Kuklin István]

Thank you very much!