Correct the expansion of $USE_PERSISTENT_DATA in CMD

Dockerfile

@@ -110,7 +110,7 @@ RUN echo "Downloading checkpoints..." && \
     # wget -c https://huggingface.co/sberbank-ai/Real-ESRGAN/resolve/main/RealESRGAN_x4.pth -P ./models/upscale_models/ && \
     echo "Done"
 
-CMD ["python", "main.py", "--listen", "0.0.0.0", "--port", "7860", "--output-directory", "${USE_PERSISTENT_DATA:+/data/}"]
+CMD python main.py --listen 0.0.0.0 --port 7860 ${USE_PERSISTENT_DATA:+--output-directory /data/}