Packer builds Docker containers without the use of Dockerfiles. By not using Dockerfiles, Packer is able to provision containers with portable scripts or configuration management systems that are not tied to Docker in any way. It also has a simple mental model: you provision containers much the same way you provision a normal virtualized or dedicated server.
From the intro: "We’ve been tracking an organized attack campaign that targets misconfigured open Docker Daemon API ports. This persistent campaign has been going on for months, with thousands of attempts taking place nearly on a daily basis."