Fix PORT environment variable substitution

Dockerfile

@@ -38,4 +38,4 @@ HEALTHCHECK --interval=30s --timeout=30s --start-period=5s --retries=3 \
     CMD curl -f http://localhost:${PORT}/ || exit 1
 
 # Run the application (uses PORT environment variable)
-CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "${PORT}"]
+CMD sh -c "uvicorn main:app --host 0.0.0.0 --port ${PORT}"