We are currently working toward removing onbuild images from the official images: docker-library/official-images#2076, since they are easily replaced with a more transparent Dockerfile and are inadequate beyond basic use cases as is the case you ran into with this issue.

So we recommend not using a Dockerfile of FROM python:onbuild and to instead be explicit:

FROM python:3.6

WORKDIR /usr/src/app

COPY requirements.txt requirements-app.txt /usr/src/app/
RUN pip install --no-cache-dir -r requirements.txt

COPY . /usr/src/app

This way you can change what needs to be copied in with or instead of requirements.txt and even make it FROM python:3.6-alpine and have more control over the size of the image by adding and removing build dependencies within the layer that they are used.