fix: use forgejo/runner as base image, add Node.js on top

- All runners now based on code.forgejo.org/forgejo/runner:6 (has runner binary)
- Dockerfile adds Node.js + specialized tools (Go, Rust, docker)
- CI workflow will build custom images and push to registry
- Image Updater will auto-detect and update values.yaml
- ArgoCD will sync new custom images when available

k8s/infra/forgejo-runner/Dockerfile.node

@@ -1,13 +1,4 @@
-FROM node:22-bookworm
+FROM code.forgejo.org/forgejo/runner:6
 
-# Install Docker client (dind sidecar provides the daemon)
-RUN apt-get update && \
-    apt-get install -y --no-install-recommends \
-      docker.io \
-      git \
-      curl \
-      ca-certificates && \
-    rm -rf /var/lib/apt/lists/*
-
-# Verify installations
-RUN docker --version && node --version && git --version
+# Node.js is already in forgejo/runner, just verify
+RUN node --version && docker --version && git --version