Fix Docker push permissions for pull requests

Only push Docker images when not in a pull request context. PRs don't have
permission to push to ghcr.io, so we should only build without pushing.

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

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

.github/workflows/deploy.yml

@@ -92,7 +92,7 @@ jobs:
         uses: docker/build-push-action@v5
         with:
           context: .
-          push: true
+          push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha