Adds support for a Route Prefix, Improves Dockerfile

[biw]

* Adds support for a route prefix.
  
  * Allows nuts to run at a `/path` vs the root of a domain or subdomain.
  * Useful for if someone has one domain they want to use but multiple projects to support updates for and are using a reverse proxy.

* Improves the Dockerfile.
  
  * Switches to Official Node Image
  * Updates to Node 6 (small bump from 5)
  * Reorganized for better layer caching and less layer creation on subsequent builds.
  * Added dumb-init for proper signal handling/passing and zombie reaping.

From GitbookIO/nuts#160