Forgejo actions base image

Docker image with all dependencies installed, configured to be as close to GitHub actions as possible.

The CI builds this image on pushes to main as git.pub.solar/pub-solar/actions-base-image:20-bookworm.

Building the image locally:

docker build --tag git.pub.solar/pub-solar/actions-base-image:20-bookworm .

Creates a ci user with passwordless sudo to enable nix single-user install in a node:20-bookworm base image.

See this GH issue for additional context.