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:$TAG.

Built and tagged on flora-6 with:

docker pull node:20-bookworm
docker build --tag node:20-bookworm .

Uses a ci user with passwordless sudo to enable nix single-user install.

See this GH issue for additional context.