Apparently nodejs-legacy has been removed since we last updated that doc. I took a stab at updating it. We also bumped the minimum required nodejs to 12 in configurable-http-proxy 4.5, mistakenly thinking node.js took that into acount at install time, like pip does.
Using nodesource is absolutely appropriate, and the best way to get the latest nodejs, but if you want to use the system node, you can pin configurable-http-proxy@^4.4 to the previous version, which still supported node 10. I’ve also proposed that we restore node 10 support in the next release, because of this.
So the absolute shortest path to jupyterhub on ubuntu 20.04 (in a Dockerfile format, showing it works from scratch):
FROM ubuntu:20.04
RUN apt-get -y update && apt-get -y install --no-install-recommends nodejs npm python3 python3-pip
RUN npm install -g 'configurable-http-proxy@^4.4' # pin version 4.4 for node 10
RUN python3 -m pip install jupyterhub jupyterlab