Add a preliminary Dockerfile

Preliminary work on a Docker version of "[How To Run Firefox Sync 1.5](https://docs.services.mozilla.com/howtos/run-sync-1.5.html)"

## Prerequisites

1. [Docker](http://docker.io)
2. [Firefox 29 (Beta)](https://www.mozilla.org/en-US/firefox/beta/), or newer

## Building and running the server

With the `Dockerfile` below in the current directory, run:

1. `docker build -t sync:server .`
2. `docker run -i -t -p 5000:5000 sync:server`
3. Visit http://localhost:5000, it should say "it works!"

## Starting / Stopping the Sync service

Use `docker ps -a` to see all containers, both running and stopped.

Use `docker stop <container_id>` to stop a running container.

Use `docker start <container_id>` to start a running container.

## Using the server

1. Go to `about:config`, change `services.sync.tokenServerURI` to `http://localhost:5000/token/1.0/sync/1.5`
2. Open the Browser Console (Ctrl-Shift-J)
3. Try to use Firefox Sync -- you should see log output from Docker, and the Browser Console should show connections to `http://localhost:5000`