Add basic flatpak github action

Author: agorgl

.github/workflows/flatpak.yml

@@ -0,0 +1,20 @@
+name: flatpak
+
+on:
+  release:
+    types: [published]
+
+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: mod-desktop.flatpak
+          manifest-path: flatpak/audio.mod.mod-desktop.yaml
+          cache-key: flatpak-builder-${{ github.sha }}