Added a Dockerfile

Hi!

This would allow people to create node-ready containers using a simple command in [docker](http://docker.io).

    docker build tools/Dockerfile

A Dockerfile contains instructions allowing you to automate the steps
you would normally manually take to create a Docker image. Docker Build
will run your steps and commit them along the way, giving you a final
image.

[More info on docker](http://docker.io)
[More info on Dockerfile](http://docs.docker.io/en/latest/use/builder)