added pip install gunicorn

Dockerfile

@@ -23,6 +23,6 @@ RUN pip install --no-cache-dir --upgrade -r ./app/requirements.txt
 # For more info, please refer to https://aka.ms/vscode-docker-python-configure-containers
 RUN adduser -u 5678 --disabled-password --gecos "" appuser && chown -R appuser /app
 USER appuser
-
+RUN pip install gunicorn
 # During debugging, this entry point will be overridden. For more information, please refer to https://aka.ms/vscode-docker-python-debug
 CMD ["gunicorn", "--bind", "0.0.0.0:8000", "-k", "uvicorn.workers.UvicornWorker", "setup:app"]