Is the node.js code being run from a container or from the host?
If it’s on the host, just use the localhost address i.e:
var url="mongodb://localhost:27017";
This will work because you published the port with -p 27017:27017.
If the code is running inside a container, it would be best to rewrite it to use links and referring to the mongo container by name e.g:
var url="mongodb://mongo:27017";
Then when you launch the container with the Node.js code, you can just do something like:
docker run -d --link mongo:mongo my_container
Docker will then add an entry to /etc/hosts inside the container so that the name mongo resolves to the IP of the mongo container.