Problem with my Docker image not running Flask - docker

Question

I am trying to build a Flask docker image. I get the error:
zsh: command not found: flask
I followed this old tutorial to get things working. https://medium.com/#rokinmaharjan/running-a-flask-application-in-docker-80191791e143
In order to just learn how to start flask website with Docker I have made everything simple. My Docker image should just open a Hello world front page.
My example.py:
from flask import Flask
app = Flask(__name__)
#app.route('/')
def hello_world():
return 'Hello World'
if __name__ == '__main__':
app.run()
My Dockerfile:
FROM ubuntu:16.04
RUN apt-get update -y
RUN apt-get install python -y
RUN apt-get install python-pip -y
RUN pip install flask
COPY example.py /home/example.py
ENTRYPOINT FLASK_APP=/home/example.py flask run --host=0.0.0.0
I run
sudo docker build . -t flask-app
to build the image.
When I run
docker run -p 8080:5000 flask-app
I get the error:
zsh: command not found: flask
What am I missing here?

Answer 1

Well, indeed you're following a really old tutorial.
I'm not going to enter into detail whether using Flask directly without a WSGI server is something you should do, so I'm just going to focus on your question.
Concise answer: you don't have the installed modules by pip in your PATH, so of course you cannot invoke them. Flask is one of this modules.
Extended answer: keep reading.
First of all, using that base image you're downloading an old version of both Python and pip, secondary: you don't need a fully fledged operative system in order to run a Flask application. There are already base images with Python like python:3.9.10-slim-buster with way less dependencies and possible vulnerabilities than an old image from Ubuntu 16.
FROM python:3.9.10-slim-buster
Second, you shouldn't rely on what do you have on the base image and you should use an environment (venv) for your application, where you can install Flask and any other dependency of the application which should be listed on the requirements.txt. Also you should choose in which working directory you would like to place your code (/usr/src/app is a common place normally).
Indicating which port are you exposing by default is also a good thing to do (even though everyone knows that Flask exposes port 5000).
FROM python:3.9.10-slim-buster
WORKDIR /usr/src/app
ENV VIRTUAL_ENV=/opt/venv
RUN python3 -m venv $VIRTUAL_ENV
ENV PATH="$VIRTUAL_ENV/bin:$PATH"
RUN python3 -m pip install flask
COPY example.py .
ENTRYPOINT FLASK_APP=example flask run --host=0.0.0.0
EXPOSE 5000
and as a result:
❯ docker run -p 8080:5000 flask-app
* Serving Flask app 'example' (lazy loading)
* Environment: production
WARNING: This is a development server. Do not use it in a production deployment.
Use a production WSGI server instead.
* Debug mode: off
* Running on all addresses.
WARNING: This is a development server. Do not use it in a production deployment.
* Running on http://172.17.0.2:5000/ (Press CTRL+C to quit)