Turn into Docker container for BIDS App

[ericearl]

Write a Dockerfile to containerize the dependencies and simplify usage as a BIDS App.

[yarikoptic]

• Gentoo based Dockerfile (presumably reproducible) to provide bids-app #41 gets you 90% of the way.
• Next step is to upload to docker hub, ideally automated procedure as well
• add entry to https://github.com/bids-apps/bids-apps.github.io/