This base image supports setting custom build options that let you modify what gets installed. You can use Docker build args to accomplish this. The currently supported options are to install PhantomJS, MongoDB, or any list of apt-get dependencies (this image is built on debian:jesse).If you choose to install Mongo, you can use it by not supplying a MONGO_URL when you run your app container. The startup script will start Mongo inside the container and tell your app to use it. If you do supply a MONGO_URL, Mongo will not be started inside the container and the external database will be used instead. (Note that having Mongo in the same container as your app is just for convenience while testing/developing. In production, you should always use a separate Mongo deployment or at least a separate Mongo container).