How to set the python root to the Dockerfile workdir

I have a simple Dockerfile:

FROM python:3.6-alpine
RUN apk add -U --no-cache gcc build-base linux-headers ca-certificates python3-dev libffi-dev libressl-dev libxslt-dev
COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt
COPY . app/
WORKDIR /app

ENV FLASK_APP app.py

EXPOSE 5000
ENTRYPOINT [ "python" ]

CMD ["-m", "flask", "run"]

When I run the app locally (not in docker) everything is fine. However, because the whole app is moved to the /app workdir, modules are not recognized anymore. I need to do this to make it work:

from app.my_module import MyClass

How do I tell python the root is /app and not /?

Source: StackOverflow