Build forgejo-runner container locally (#334)

## Summary
- Add native Dagger `container.py` for forgejo-runner (Go + Alpine runtime, static binary with CGO for SQLite)
- Update kustomization to point to local registry image (tag is placeholder until CI builds)
- Uses existing `clone_from_forge("forgejo-runner", ...)` mirror

## Test plan
- [x] `dagger call build --src=. --container-name=forgejo-runner` passes locally
- [ ] CI container build from branch succeeds
- [ ] Update kustomization tag to built image, deploy from branch via ArgoCD `--revision`
- [ ] Verify runner registers and picks up jobs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #334

argocd/manifests/forgejo-runner/kustomization.yaml

@@ -10,7 +10,8 @@ resources:
 
 images:
   - name: code.forgejo.org/forgejo/runner
-    newTag: "12.7.3"
+    newName: registry.ops.eblu.me/blumeops/forgejo-runner
+    newTag: v12.7.3-12d83ba
   - name: docker
     newTag: 27-dind