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.golang

@@ -1,15 +1,10 @@
-FROM docker:27-cli
+FROM code.forgejo.org/forgejo/runner:6
 
-# Install Node.js + build essentials (needed for GitHub Actions checkout@v4, etc.)
+# Install Node.js (needed for GitHub Actions checkout@v4, etc.)
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
       nodejs \
-      npm \
-      git \
-      curl \
-      ca-certificates \
-      build-essential \
-      python3 && \
+      npm && \
     rm -rf /var/lib/apt/lists/*
 
 # Verify installations