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-node.yaml

@@ -4,13 +4,13 @@
 #
 # node:22-bookworm ships Node natively. Docker client installed via workflow step if needed.
 # (homelab has no CI; custom runner images built manually if desired)
-# Bootstrap with base image, custom image via CI+Image Updater
+# Bootstrap with runner image (already has Node.js), CI builds custom
 runner:
   image:
-    repository: node
-    tag: "22-bookworm"
+    repository: code.forgejo.org/forgejo/runner
+    tag: "6"
   name: node-runner
-  labels: "node:docker://node:22-bookworm"
+  labels: "node:docker://code.forgejo.org/forgejo/runner:6"
 
 # GC CronJob renders only from the default (golang) values to avoid duplicates
 gc: