How to translate flask run to Dockerfile? - dockerfile

Question

So I'm trying to learn how to containerize flask apps and so far I've understood two ways of firing up a flask app locally:
One is to have this code in the main file:
if __name__ == '__main__':
APP.run(host='0.0.0.0', port=8080, debug=False)
and run with
python3 main.py
The other is to remove the above code from main.py, and just define an environment variable and do flask run:
export FLASK_APP=main.py
flask run
I tried to convert both of these methods into a Dockerfile:
ENTRYPOINT["python3", "main.py"]
which works quite well for the first. However when I try to do something like:
ENV FLASK_APP "./app/main.py"
ENTRYPOINT ["flask", "run"]
I am not able to reach my server via the browser. The container starts up all well, just that it's not reachable. One trick that does work, is if I add the host address in the entrypoint like so:
ENTRYPOINT ["flask", "run", "--host=0.0.0.0"]
I am not sure why do I have to the --host to the entrypoint when locally I can do also without it. Another funny thing that I noticed, was that if I put the host as --host=127.0.0.1, it still doesn't work.
Can someone explain what really is happening here? Either I don't understand the ENTRYPOINT correctly or maybe flask.. or maybe both.
EDIT: The whole Dockerfile for reference is:
FROM python:stretch
COPY . /app
WORKDIR /app
RUN pip3 install --upgrade pip
RUN pip3 install -r requirements.txt
ENV FLASK_APP "/app/main.py"
ENTRYPOINT ["flask", "run", "--host=127.0.0.1"]

Answer 1

Try to define FLASK_APP env via absolute path.
Or put this string to your Dockerfile:
WORKDIR /path/to/dir/that/contains/main.py/file

Answer 2

Oh sorry. Host in ENTRYPOINT statement must be 0.0.0.0 :
ENTRYPOINT ["flask", "run", "--host=0.0.0.0"]
And do not forget to tie 5000 port outside via -p option :
docker run -p 5000:5000 <your container>

Answer 3

i believe this task would be better accomplished implementing CMD as opposed to ENTRYPOINT.
(you should also define the working directory before running COPY command.)
for example, your Dockerfile should look something like..
FROM python:stretch
WORKDIR /app
COPY . .
RUN pip3 install --upgrade pip
RUN pip3 install -r requirements.txt
CMD python3 main.py