This container allows for easy maintenance of a Portage overlay on a non-Gentoo system. It can also be used as a throwaway fresh install that you can use to verify that your ebuilds include all dependencies. It sets up a Gentoo container, pulls in an overlay, and adds a few tools to facilitate maintenance. It's set up to work with my overlay, but editing the Dockerfile should allow you to work with any overlay you want.

Copy the SSH keys you need to access your Git repo into the ssh-keys directory, then build the image with docker compose build. To use, run the image with docker compose run -it --rm gentoo. If you want the image to persist a while, you could start it with docker compose run -d --rm --name gentoo gentoo tail -f /dev/null, do what you need within it with docker exec -it gentoo bash, and dispose of it with docker stop gentoo.