How to get npm 5.6.0 while running ubuntu:16.04 container

My configuration:
Ubuntu 16.04
npm: 5.6.0
node: 8.0.0

I entered the command lines:
docker pull ubuntu:16.04
docker run -it ubuntu:16.04
then, as a root:

apt-get update
apt-get instal npm

and after the installation succeeded,

npm install npm@latest

The problem is that, when I enter, while running ubuntu:16:04 container,
npm -v

I only have the version 3.5.2
How can I upgrade the npm and node versions inside the image?

I need more recent versions (I would like to get the npm 5.6.0 version) in order to be able to build another project inside the ubuntu image.
Thanks in advance!

see
https://www.godaddy.com/help/install-nodejs-ubuntu-17395