* Use debian base container and clean up apt-get
Attempt to slim down the docker image by replacing ubuntu:trusty with debian:jessie and clean up after apt-get invocation. Building against the 3.6.0 commit shows a healthy reduction of 44MB, or 9%, compared to the ubuntu-based image.
* Use debian:jessie-slim for an even smaller image
If we're using a debian base image for sizing reasons, we may as well go the full distance and use debian-slim.