Periodic cron for package/security list updates causing hanging issues on docker

The upstream Ubuntu docker image has periodic update enabled (i.e. AutoAptEnable APT::Periodic::Enable=1) which can cause issues when extending the image with other tools that also use cron for one purpose or another. By itself systemd/cron is not installed.

In my scenario I had a team add Nginx that ran cron on startup and periodically to do maintenance work. This brought in cron (for better or worse).

With cron now in place to do Nginx related things it also started processing the APT related cron which done in conjunction with Nginx would cause CPU spikes (due to intentional resource limitations) and made apt-config hang in the container, which also makes the container often hang.

See https://github.com/tianon/docker-brew-ubuntu-core/pull/137 for original issue where I proposed to disable APT::Periodic:Enable) in Docker images as docker images should be immutable; any updates to packages or apt list should happen in a build system.