Removes custom Docker lookup code and instead integrates it with the generic lookup routine used by other package managers. Logic for digest support was added but is used by Docker-only for now.
Closes#2081, Closes#2276
Manually finds and massages node updates in Docker, Docker Compose and Circle CI so that they should take on the same “renovate/node-8.x” style branch naming. The goal is to unify all node updates into a single branch.