feat(ci): update Dockerfile and CI configuration for backend image and caching

Signed-off-by: Noooste <83548733+Noooste@users.noreply.github.com>

.github/workflows/test.yml

@@ -27,7 +27,7 @@ jobs:
 
       - name: Generate swagger docs
         run: |
-          go install github.com/swaggo/swag/cmd/swag@latest
+          go install github.com/swaggo/swag/cmd/swag@v1.16.4
           cd backend && swag init
 
       - name: Run unit tests with race detector and coverage
@@ -66,6 +66,19 @@ jobs:
           cache: true
           cache-dependency-path: backend/go.sum
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Pre-build backend image with layer cache
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: Dockerfile
+          load: true
+          tags: garage-ui-smoke-backend:ci
+          cache-from: type=gha,scope=smoke-backend
+          cache-to: type=gha,mode=max,scope=smoke-backend
+
       - name: Run smoke test
         run: make test-smoke