docker compose Flask app returning empty response

I am trying to play with a dockerized flask container using docker-compose. The flask app is a hello world app that works correctly when I test it on my host.

The docker-compose file looks like this:

version: '3'
services:
  web:
    image: ubuntu
    build:
      context: ./
      dockerfile: Dockerfile
    ports:
      - "6000:5000"

My dockerfile looks like this:

FROM ubuntu

RUN apt-get update
RUN apt-get install -y python3 python3-pip

RUN pip3 install Flask

COPY app.py .

EXPOSE 5000

CMD ["python3", "app.py"]

The hello world app looks like this:

from flask import Flask, request

app = Flask(__name__)

@app.route("/")
def index():
    return "Hello, World!"

if __name__ == "__main__":
    app.run(debug=True)

When I bring up the containers using docker-compose up -d, there is no error. When I curl to localhost:6000, I get this :

curl -X PUT localhost:6000
curl: (52) Empty reply from server

It seems like the app is responding but not how it responds when I run it on a my host and just returns an empty string instead of "hello world" when I curl to it. What am I doing wrong?

There are several issues here:

1. You need to make your flask app accessible externally: app.run(host="0.0.0.0", debug=True).
2. You are sending a PUT request from curl but your app only allows GET (this is the default, you can add allowed methods to your route like this: @app.route("/", methods=["GET", "PUT"]).
3. This one is not actually a bug but rather a recommendation: don't use port 6000 if you need your app to be accessible via web browsers. At least some of them will display an ERR_UNSAFE_PORT error and will block the request. It will work via curl though.