Add Github action for building Flatpak

.github/workflows/flatpak-ci.yml

@@ -0,0 +1,21 @@
+name: Flatpak CI Tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  flatpak:
+    name: "Flatpak"
+    runs-on: ubuntu-latest
+    container:
+      image: bilelmoussaoui/flatpak-github-actions:kde-5.15-23.08
+      options: --privileged
+    steps:
+    - uses: actions/checkout@v4
+    - uses: flatpak/flatpak-github-actions/flatpak-builder@v6
+      with:
+        bundle: yin-yang.flatpak
+        manifest-path: sh.oskar.yin-yang.json
+        cache-key: flatpak-builder-${{ github.sha }}