Document that pentext-docker exists

[ros-cr]

We have https://github.com/radicallyopensecurity/pentext-docker as a combination of Docker files, GitLab integration, template conversion programs and other glue logic.

I think it would be very relevant to add to Pentext's documentation that pentext-docker exists, and give some hints at what it does. As I understand it @PeterMosmans aims to keep the pentext repository separate from additional software layers and tooling like pentext-docker, and some of pentext-docker is ROS- and CI/CD specific and not useful to the average person. Still, I think it's relevant to let interested readers of the README.md know that this additional software exists, since they likely won't find out otherwise.

To give one example that's probably relevant for individuals interested in pentext: the docbuilder logic in pentext-docker is very useful to convert (= render) pentext documents during local document editing, not just in a CI context. It's not 100% set up for that role and doesn't make use of various local features (e.g., rootless Podman usage, rebuilding on file changes), but it's a better place to start than re-doing the whole container setup on their own 🙂