Create a docker-based build.

This helps make it easier to run this build by removing the necessity to
setup and run a local mongo server.
It can't be null, as we use this same variable when constructing the Queue object (and it throws an exception if the URL isn't a string).  I think it's better for the URL variable to have an actual URL.  I like it more with the port on there (if someone is getting failures running the build because of connecting to the wrong port it helps make it easier for them to spot the mistake), but if you can give me a good reason to remove it I can do so.