Base node Docker images

Now based on the official ubuntu-based images and alpine-python multiarch approach, but targeting only LTS releases.

Current tags:

• insightful/ubuntu-node:latest, which is a virtual image for
  • insightful/ubuntu-node:14-amd64 (currently at v14.14.0)
  • insightful/ubuntu-node:14-arm32v7
  • insightful/ubuntu-node:14-arm64v8

All tags have a full install with npm and yarn.