We really need a local NPM cache

[yaronyg]

Today we lost a whole day of productivity because NPM wasn't behaving itself. We really need CI to have its own NPM registry so we don't have problems like this. Something like https://www.npmjs.com/package/sinopia should be super easy to set up.

[yaronyg]

At this point what we need to know is if Sinopia is able to function properly offline and if it will always download a local cached version of the tgz file.