Add post `apt-get install` cleanup
Implements best practices from https://docs.docker.com/engine/articles/dockerfile_best-practices/#apt-get
Fixes #974 (closed)
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Implements best practices from https://docs.docker.com/engine/articles/dockerfile_best-practices/#apt-get
Fixes #974 (closed)