Speed up nodejs installation

[nomadster]

Hi,
I've created a pull request (#7) to speed up the installation of nodejs.
It uses apt-get to install npm which allows us to install n in roder to be able to have nodejs-latest already compiled.

Also, it depends on dockerfile/ubuntu instead of dockerfile/python (why do you need python?)
It also has an ENV variable that allows you to select which nodejs version you want to install.

I've seen that in #3 someone suggest to use tagging, which I still have to investigate, but seems a pretty way to specify what version of node to install