Add Docker image builder

I've created a Docker image builder, using Packer, along with a Gradle build runner to parallelize. It creates Docker images for: CentOS 7.6, RHEL 7.6, Ubuntu 18.04, Ubuntu 16.04.

https://github.com/apolloclark/packer-cloudsploit

It runs great, takes only a couple of minutes to build all 5 images.

Start by reviewing the proposed implementation at https://github.com/apolloclark/packer-cloudsploit and compare it with the repository's current build setup. The requested result is a Docker image builder that produces images for CentOS 7.6, RHEL 7.6, Ubuntu 18.04, and Ubuntu 16.04, with parallelized builds through the described Gradle runner.

Written by the indexing model from the issue text.