ci: Add Docker build check to catch Dockerfile issues early

- Add docker-build job that runs on every push/PR
- Builds only amd64 platform for speed (vs multi-platform in release)
- Uses GitHub Actions cache for faster builds
- Prevents Dockerfile issues from reaching release workflow

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -73,6 +73,27 @@ jobs:
       - name: Run tests
         run: cargo nextest run --verbose
 
+  docker-build:
+    name: Docker Build Check
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build Docker image (amd64)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64
+          push: false
+          tags: barycenter:ci-test
+          cache-from: type=gha,scope=ci-docker-amd64
+          cache-to: type=gha,mode=max,scope=ci-docker-amd64
+
   security:
     name: Security Audit
     runs-on: ubuntu-latest