fix: use docker driver for buildx on unprivileged runner (#219)

The build-push-action was trying to use docker-container driver by default,
which requires privileged mount operations. This fails on unprivileged runners
with vfs storage driver.

Solution: Explicitly configure buildx to use 'docker' driver, which uses the
regular Docker daemon that's already running with vfs storage. This avoids
the need for privileged container mounts.

Resolves: operation not permitted during buildkit mount

Reviewed-on: graham/towerops-web#219

.forgejo/workflows/production.yaml

@@ -120,6 +120,12 @@ jobs:
           username: ${{ secrets.REGISTRY_USER }}
           password: ${{ secrets.REGISTRY_PASSWORD }}
 
+      - name: Set up Docker Buildx
+        uses: https://github.com/docker/setup-buildx-action@v3
+        with:
+          driver: docker
+          install: true
+
       - name: Build and push Docker image
         uses: https://github.com/docker/build-push-action@v5
         with: