How to set up Flask+Gunicorn+Nginx using docker-compose?

I have Flask app with couple API endpoints: /api1 and /api2

Flask (flask_app.py):

app = Flask(__name__)
...
@app.route('/api1', methods=['POST'])
def api1():
...
@app.route('/api2', methods=['POST'])
def api2():
...
if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

Dockerfile:

...
# Copy application code
COPY flaskapp /app
WORKDIR /app

# Set entrypoint
ENTRYPOINT ["gunicorn", "-b", "0.0.0.0:5000", "flask_app:app"]

docker-compose:

services:

  nginx:
    image: nginx:latest
    container_name: nginx_container
    restart: always
    ports:
      - "80:80"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
    depends_on:
      - app

  app:
    build:
      context: .
      dockerfile: Dockerfile
    container_name: flask_container
    restart: always
    ports:
      - "5000:5000"
    volumes:
      - ./flaskapp:/app

nginx.conf:

events { 
    worker_connections 1024; 
}
http {
    client_max_body_size 0;
    server {
        listen 80;
            location / {
                proxy_pass http://flask_container:5000/;
            }
        client_max_body_size 0;
    }
}

I run it with Gunicorn and Nginx using docker-compose up. Everything starts properly. Then I test it using Postman sending POST requests on this urls:

http://localhost:5000/api1
http://localhost:80/api1

but Postman returns 500 error every time.

Request to http://localhost:80/api1 also writes this message to Nginx console:

172.18.0.1 - - [08/Jun/2023:18:26:41 +0000] "POST /api1 HTTP/1.1" 500 265 "-" "PostmanRuntime/7.32.2"

I expected that POST requests will go to Flask API. Why it doesn't?

500 error code meant error was happening on API side during code execution.

For debugging could be used app.run(debug=True) or logging library in python.