Dockerize nationalmap

This adds docker config for the node application server, and varnish. I've also updated config.js to interrogate the environment for config injected by the docker environment regarding mongo.

The application server image is build directly on top of a standardized node language stack image, with the addition of gdal-bin. The varnish image is hand rolled and contains a custom boot script in place of the upstart script provided by the package maintainers.

A quick howto:
```bash
# From the nm repo root directory, with a working nm build.
# Build the application server image.
sudo docker build -t nationalmap/backend .

# Build the varnish image.
sudo docker build -t nationalmap/varnish varnish/

# Boot up the official mongodb image.
sudo docker run -d -p 27017:27017 --name mongodb dockerfile/mongodb

# Start the backend image you built.
sudo docker run --name nationalmap --link mongodb:mongodb nationalmap/backend

# Start the varnish image you built.
sudo docker run  -d -p 80:80 --name varnish --link nationalmap:nm nationalmap/varnish
```

I'll add a complete set of documentation to the wiki once the pr is accepted.