You generally don't install anything to the node-red root. You install packages containing nodes to the userDir folder. That defaults to ~/.node-red on a "standard" install. For the standard Docker build, that folder is /data/ as stated in the docs Running under Docker : Node-RED (nodered.org).
Those same docs show you how to get into your docker instance as a terminal.
Further down that same page, it tells you how you can have your own custom /data/ folder which is what you are wanting to do I think.
