build: add docker build step to test workflow

.forgejo/workflows/test.yml

@@ -8,6 +8,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+        with:
+          driver: remote tcp://docker-in-docker:2375
+      - name: docker build
+        uses: docker/build-push-action@v6
+        with:
+          push: false
+
       - name: Test
         uses: https://git.kemitix.net/kemitix/rust@next
         with: