I develop a flask application in a docker container. A minimal example of my code has the following structure:

+- myproject
   +- Flask-App
      +- src
         +- __init__.py
      +-__init__.py
      +- Dockerfile
      +- requirements.txt
      +- wsgi.py
+- docker-compose.yml

I made the example even more minimalistic (see second edit)

Since, I am currently developing the application, I want to add src as a volume to my container. Without mounting the volume, I have to rebuild the container every time. Whenever I add the volume, this error is raised:

Attaching to flask
flask    | Traceback (most recent call last):
flask    |   File "wsgi.py", line 1, in <module>
flask    |     from src import create_app
flask    | ImportError: cannot import name 'create_app' from 'src' (unknown location)
flask exited with code 1

Without the volume command in docker-compose.yml, the code runs. Can you please explain how I can circumvent this problem?

docker-compose.yml

version: '3.1'

services:
  flask:
    build:
      context: ./Flask-App
      dockerfile: Dockerfile
    container_name: flask
    ports:
     - "5000:5000"
    volumes:                      # produces error
      - ./src/:/home/appuser/src/ # produces error

Flask-App/wsgi.py

from src import create_app

app = create_app()

if __name__ == '__main__':
   app.run(host='0.0.0.0', port=5000)

Flask-App/requirements.txt

Flask==1.1.1

Flask-App/Dockerfile

FROM python:3.7.3-stretch

COPY ./requirements.txt /tmp/
RUN pip install -r /tmp/requirements.txt

RUN useradd --home /home/appuser appuser

WORKDIR /home/appuser
USER appuser

COPY ./ ./

# Run the application
ENTRYPOINT ["python"]
CMD ["wsgi.py"]

Flask-App/src/__init__.py

from flask import Flask


def create_app():

    app = Flask(__name__)

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

    return app

EDIT 1

Flask-App/Dockerfile

FROM python:3.7.3-stretch

COPY ./requirements.txt /tmp/
RUN pip install -r /tmp/requirements.txt

RUN useradd --home /home/appuser appuser

WORKDIR /home/appuser
USER appuser

COPY ./ ./
RUN echo $(ls -alt /home/appuser)  # newly added
# Run the application
ENTRYPOINT ["python"]
CMD ["wsgi.py"]

Output of echo ls

total 24
drwxr-xr-x 1 root root 4096 Oct 22 11:55 .
-rwxr-xr-x 1 root root 345 Oct 22 11:55 Dockerfile
-rwxr-xr-x 1 root root 117 Oct 22 11:25 wsgi.py
-rwxr-xr-x 1 root root 0 Oct 22 11:17 __init__.py
drwxr-xr-x 2 root root 4096 Oct 22 11:17 src 
drwxr-xr-x 1 root root 4096 Oct 21 11:15 ..
-rwxr-xr-x 1 root root 12 Oct 21 11:14 requirements.txt

EDIT 2

Ok, I made it even more minimalistic. This example does not need Flask at all. When I mount the volume, the program raises again an error

flask_1  | Traceback (most recent call last):
flask_1  |   File "app.py", line 1, in <module>
flask_1  |     from src import create_app
flask_1  | ImportError: cannot import name 'create_app' from 'src' (unknown location)

Below, I added again all the files to reproduce the error.

docker-compose.yml

version: '3'

services:
  flask:
    build:
      context: ./Flask-App
    volumes:
      - ./Flask-App/src:/usr/src/app/sr

Flask-App/src/__init__.py

class MyTest:

    def my_print_fnc(self):
        print("Hello World")


def create_app():
    return MyTest()

Flask-App/app.py (replaces wsgi.py)

from src import create_app

app = create_app()

if __name__ == '__main__':
    app.my_print_fnc()

Flask-App/Dockerfile

FROM python:3.7.3-stretch

COPY . /usr/src/app

WORKDIR /usr/src/app


# Run the application
ENTRYPOINT ["python", "app.py"]

docker-compose.yml

version: '3'

services:
  flask:
    build:
      context: ./Flask-App
    volumes:
      - ./Flask-App/src:/usr/src/app/src