Build and publish GitHub runner image to GHCR - #4

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8, docker/metadata-action@c299e40c65443455700f0fdfc63efafe5b349051, docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/