Fix: Add default port value when PORT env var is empty

Dockerfile

@@ -48,6 +48,6 @@ HEALTHCHECK --interval=30s --timeout=10s --start-period=40s --retries=3 \
     CMD curl --fail http://localhost:${PORT:-7860}/_stcore/health || exit 1
 
 # Run Streamlit app using PORT env var from Hugging Face Spaces
-# HuggingFace sets $PORT, don't override it.
+# HuggingFace sets $PORT, don't override it. Default to 7860 if not set.
 # Temporarily using app_test.py for debugging
-CMD ["bash", "-c", "echo Using PORT=$PORT && streamlit run app_test.py --server.address=0.0.0.0 --server.port=$PORT"]
+CMD ["bash", "-c", "echo Using PORT=${PORT:-7860} && streamlit run app_test.py --server.address=0.0.0.0 --server.port=${PORT:-7860}"]