Each *.Dockerfile generates an image based on the operating system in the name and containing a minimum amount of additional software but for Docker and the software to install and auto-join runner agents. Additional software installs are controlled by an adjacent operating-system.sh script. That script mostly calls scripts in images/software/ to actually install whatever software is wanted.
The goal is that ubuntu-latest (focal or 20.04) is as close to the ubuntu-latest labelled runner in GitHub.com as is requested on a feature-by-feature basis from our users. I've been assuming that this is the default runner for most users. The GitHub repository for GitHub's hosted runners is here and it's awesome! However, it is also gigantic and pretty specific to Azure, so while I'll shamelessly pilfer the software management methodology, it's not going to work for us.
Images are built and tested at pull requests against the main branch automatically.
Images are built and pushed once those pull requests have been merged into the main branch by GitHub Actions.
You can do a lot with the deployment aspect of this project. To keep it simple, I set the runner controller to always pull a fresh image and to ensure the pods are ephemeral. This isn't exactly continuous deployment, but I've found it works well enough.
If you'd like to build these manually on your own laptop, it's pretty straightforward. Just meander into the images directory and run a Docker build like shown below.
cd images/
docker login docker.your-ghe-url
docker build -f image-name.Dockerfile -t tag-goes-here .
docker push tag-goes-here