title: Docker cannot find file when building the image
body: <p>This is my Dockerfile</p>
  
  <pre><code>FROM python:3.6-slim
  
  COPY app/ /app
  
  EXPOSE 8000
  
  RUN cd /app/
  RUN pip install -r requirements.txt
  
  WORKDIR /app/project/
  
  RUN python manage.py makemigrations
  RUN python manage.py migrate
  RUN export DJANGO_SETTINGS_MODULE="project.settings"
  
  CMD [ "python", "manage.py", "server"]
  </code></pre>
  
  <p>When I try to build it, it fails saying that it can't find the requirements.txt inside the app folder.</p>