bug 957802: Combine commands in KumaScript image
bug 957802: Combine commands in KumaScript image

* Set ownership when files are copied rather than as a second step
* Combine the npm install into a single Docker layer
* Fix spelling and re-word some comments

SphinxKnight/kuma,a2sheppy/kuma,a2sheppy/kuma,mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,escattone/kuma,escattone/kuma,escattone/kuma,Elchi3/kuma,mozilla/kuma,a2sheppy/kuma,Elchi3/kuma,SphinxKnight/kuma,mozilla/kuma,mozilla/kuma,Elchi3/kuma,Elchi3/kuma,mozilla/kuma,SphinxKnight/kuma,SphinxKnight/kuma,SphinxKnight/kuma,Elchi3/kuma,a2sheppy/kuma