fix: add Docker Buildx setup with docker-container driver (#214)

The docker/build-push-action@v5 requires proper BuildKit configuration
to work on unprivileged CI runners. The setup-buildx-action with the
docker-container driver creates a properly configured BuildKit builder
that can handle the mount operations required for builds.

This restores the configuration that was working before the Gleam
removal changes.

Reviewed-on: graham/towerops-web#214

.forgejo/workflows/production.yaml

@@ -103,6 +103,15 @@ jobs:
             timeout 30 bash -c 'until docker info > /dev/null 2>&1; do sleep 1; done'
           fi
 
+      - name: Set up Docker Buildx
+        uses: https://github.com/docker/setup-buildx-action@v3
+        with:
+          driver: docker-container
+          driver-opts: |
+            image=moby/buildkit:latest
+          install: true
+          cleanup: true
+
       - name: Generate image tag
         id: tag
         run: |