add github action to build apk on release (#60)

.github/workflows/release.yml

@@ -0,0 +1,31 @@
+name: Release Flutter APK
+
+on:
+  release:
+    types: [created]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+
+    - name: Setup Flutter
+      uses: subosito/flutter-action@v2
+      with: 
+        flutter-version: '3.22.0'
+
+    - name: Install dependencies
+      run: flutter pub get
+    - name: Generate l10n
+      run: flutter gen-l10n
+    - name: Build APK
+      run: flutter build apk --release
+
+    - name: Upload APK
+      uses: actions/upload-artifact@v2
+      with:
+        name: run-flutter-run.apk
+        path: build/app/outputs/flutter-apk/app-release.apk