ci: Add on-demand Helm publish workflow

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

.github/workflows/helm.yaml

@@ -0,0 +1,15 @@
+name: helm
+
+on:
+  workflow_dispatch:
+
+jobs:
+  build-push:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Publish Helm charts
+        uses: stefanprodan/helm-gh-pages@v1.3.0
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          charts_url: https://flagger.app