Docker [WIP]

*This branch is a work-in-progress, so this is PR is a placeholder for a conversation, not a request to merge.*

This adds a basic Dockerfile, currently tuned for use in development.

It would be nice to push binary images of OpenSpending, suitable for production usage, to the [public docker registry](https://index.docker.io/), allowing people to deploy OpenSpending with a command as simple as `docker run openspending/openspending`.

At the moment, this `Dockerfile` builds a development-friendly OpenSpending, that allows the mounting of the source directory at `/src`. Perhaps we should use `Dockerfile.dev` for development and put sane defaults for production in `Dockerfile`.

I've also wondered about including a [`fig.yml`](http://orchardup.github.io/fig/index.html) in the repository...

Thoughts? To be clear this currently duplicates some of the functionality of @pudo's Vagrantfile, but is somewhat more flexible in that it can be used both as part of a one-click development environment, *and* for deployment in production. Eventually I think we should standardise on one or the other approach to configuring a dev env.