Document how to use and extend the cijoe docker image

[safl]

Docker Image for cijoe

A Docker image is now available at https://github.com/refenv/cijoe/pkgs/container/cijoe-docker
This image includes everything needed to build, install, and run cijoe, along with all tools required by its built-in packages. Especially nice is the availability of docker (for dinD) and qemu.

Documentation should describe how to utilize this image to maximize its benefits in GitHub Actions workflows.