Changed base image to phusion/baseimage:0.9.15

The previous `entry_point.sh` script did not properly track and shutdown
child processes causing container teardown to reach the default docker
timeout before forcefully terminating the container. Changing to the
`phusion/baseimage` base and running processes through runit will ensure
that processes stay running as well as shutdown gracefully.

See http://phusion.github.io/baseimage-docker/ for further details and
explanation of the benefits involved with the change.