Add Dockerfile

[wololock]

Attached Dockerfile allows to create small docker image (~50 MB) that runs doorman. It's quite useful if you run multiple microservices using docker and you want integrate docker into your environment.

[mnutt]

Thanks for the contribution. I see the value in having a slimmer base image, but think most people will have an easier time getting started with the official node base. But it certainly makes sense to have busybox-based doorman builds in the docker registry if you'd like to push one up.