[Docker] Move to alpine based image

Hi,

First, thank for sharing your code with us. I was wondering why you use a node:6 images as based image since its weight is 654MB which is pretty heavy.
I would suggest to move to an alpine based such as [`mhart/alpine-node`](https://hub.docker.com/r/mhart/alpine-node/) which weight 49,08MB.

By the way, the "david" image doesn't seems to be on docker hub.