Adds the Dockerfile (FROM code.forgejo.org/forgejo/runner:12.11.1, plus
node/git/bash/coreutils/curl/tar/make/ca-certificates) and a README with
the multi-arch build/push workflow. Built and pushed as
gitea.langenoja.se/zegit-zoo/zegit-forgejo-runner:12.11.1-ci; consumed by
flux-config applications/forgejo-runner.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
