[CS] Get all dependency downloading cached
[CS] Get all dependency downloading cached

The "onbuild" version of the node docker image was forcing us to
download some dependencies on every build, since it happened after
copying code over.

This moves to the unadorned node base image, and handles all dependency
downloading before the `COPY . /usr/src/app` directive, so they will be
cached for the vast majority of new builds.

votinginfoproject/Metis,votinginfoproject/Metis,votinginfoproject/Metis