Updated for consistency with phusion/baseimage boilerplate and lock down Erlang version.

- Comply with phusion/baseimage boilerplate setup. Especially running
init scripts.
- Use ‘ENV REFRESHED_AT’ var per The Docker Book to periodically force
refresh of underlying images so things like apt-get update won’t go
stale.
- Lock down the Erlang version to 1:17.1 so builds are consistent and
explicit.
- Install current hex and rebar local scripts.
- Run apt-get update more quietly with -qq
- Run wget quietly
- Remove no longer needed Precompiled Elixir zip file.
- Clean up apt-get and tmp dirs
- Add additional comments and links for learning more.
- Show example usage for running one-off scripts with phusion/baseimage
- Provide commented out line for disabling SSH for those who don’t want
it.
- Update README.md with link to phusion/baseimage info.