bump up docker/build-push-action action to v6

Author: itchyny

.github/workflows/ci.yaml

@@ -114,7 +114,7 @@ jobs:
           username: ${{ github.repository_owner }}
           password: ${{ secrets.GITHUB_TOKEN }}
       - name: Build and release Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: ${{ startsWith(github.ref, 'refs/tags/v') }}