npm command not available in Docker container

I use this:

RUN sudo apt-get install -y nodejs

and then we link nodejs executable to node like so:

RUN sudo ln -s `which nodejs` /usr/bin/node

that's all well and good, but for some reason the "npm" command is not available after installing nodejs.

Why would that be? I thought npm was always bundled with nodejs? What do I need to do to get npm installed too? I am certain that nodejs is installed, but npm does not appear to be present, $(which npm) yields nothing.

via Alexander Mills