ci: add Flatpak builder job

.github/workflows/main.yml

@@ -25,7 +25,7 @@ jobs:
       with:
         args: "pylint src/vcs/"
 
-  build:
+  meson-build-test:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
@@ -44,3 +44,18 @@ jobs:
       uses: BSFishy/meson-build@v1.0.3
       with:
         action: test
+
+  flatpak-build-test:
+    runs-on: ubuntu-latest
+    container:
+      image: bilelmoussaoui/flatpak-github-actions:gnome-40
+      options: --privileged
+    steps:
+    - uses: actions/checkout@v2
+    - name: Flatpak builder
+      uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v3
+      with:
+        bundle: "diffuse.flatpak"
+        manifest-path: "io.github.mightycreak.Diffuse.yml"
+        cache-key: flatpak-builder-${{ github.sha }}
+        run-tests: "true"