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-rust.yaml

@@ -2,13 +2,13 @@
 # runner instance. Only runner.name and runner.labels differ -- everything
 # else (image, dind, persistence, tolerations, nodeSelector) is shared.
 #
-# Bootstrap with base image, custom image via CI+Image Updater
+# Bootstrap with runner image, CI builds custom with Node.js+Rust
 runner:
   image:
-    repository: docker
-    tag: "27-cli"
+    repository: code.forgejo.org/forgejo/runner
+    tag: "6"
   name: rust-runner
-  labels: "rust:docker://docker:27-cli"
+  labels: "rust:docker://code.forgejo.org/forgejo/runner:6"