🔖 Add release workflow

.github/workflows/release.yaml

@@ -0,0 +1,18 @@
+name: Create Flutter app release
+on:
+  workflow_dispatch:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./app
+    steps:
+      - uses: actions/checkout@v2
+      - uses: subosito/flutter-action@v1
+      - run: flutter pub get
+      - run: flutter build apk
+      - uses: actions/upload-artifact@v1
+        with:
+          name: release-apk
+          path: build/app/outputs/apk/release/app-release.apk