ci: add runner smoke test

.forgejo/workflows/smoke.yaml

@@ -0,0 +1,10 @@
+name: runner-smoke
+on:
+  workflow_dispatch: {}
+jobs:
+  smoke:
+    runs-on: [k8s]
+    steps:
+      - run: |
+          echo "hello from $(uname -m) runner"
+          docker info 2>/dev/null | head -3