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/values.yaml

@@ -1,9 +1,9 @@
 runner:
   image:
-    repository: docker
-    tag: "27-cli"  # Bootstrap with base image, custom image via CI+Image Updater
+    repository: code.forgejo.org/forgejo/runner
+    tag: "6"  # Bootstrap with runner image, CI builds custom with Node.js
   name: golang-runner
-  labels: "golang:docker://docker:27-cli"
+  labels: "golang:docker://code.forgejo.org/forgejo/runner:6"
   forgejoUrl: http://forgejo-gitea-http.cicd.svc.cluster.local:3000
   tokenSecret: runner-token
   resources: