ci: Build docker image before running tests

.github/workflows/dart.yml

@@ -28,6 +28,8 @@ jobs:
         uses: dart-lang/setup-dart@v1
       - name: Setup
         run: ./tool/setup.sh
+      - name: Build test Docker image
+        run: ./tool/build-dev-container.sh
 
       - name: Check formatting
         run: melos run format-check