Add Yarn to Node.JS image

[alexandrevicenzi]

The Node.JS image in DockerHub includes Yarn, perhaps it makes sense to also include it in our image by default.

[dcermak]

We don't have yarn in sles iirc

[alexandrevicenzi]

I noticed that we don't have a Yarn package, but if we agree that this is indeed useful we could potentially submit Yarn to Factory and SLE.