Change installation method for node in Dockerfile

[VladNastase]

The current method used for installing node is deprecated, showing this warning everytime a container is built:

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                           SCRIPT DEPRECATION WARNING


  This script, located at https://deb.nodesource.com/setup_X, used to
  install Node.js is deprecated now and will eventually be made inactive.

  Please visit the NodeSource distributions Github and follow the
  instructions to migrate your repo.
  https://github.com/nodesource/distributions

  The NodeSource Node.js Linux distributions GitHub repository contains
  information about which versions of Node.js and which Linux distributions
  are supported and how to install it.
  https://github.com/nodesource/distributions


                          SCRIPT DEPRECATION WARNING

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TO AVOID THIS WAIT MIGRATE THE SCRIPT
Continuing in 60 seconds (press Ctrl-C to abort) ...

This also makes the build take 60 seconds longer.
It should be updated by following these instructions.