This should do it.
You need to install the build dependencies,
zlib-dev,
jpeg-dev,
gcc, and musl-dev
# pull the official base image
FROM python:3.9.6-alpine
# set work directory
WORKDIR /usr/src/app
# set environment variables
ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1
# install dependencies
RUN apk add -u zlib-dev jpeg-dev gcc musl-dev
RUN python3 -m pip install --upgrade pip
COPY ./requirements.txt /usr/src/app
RUN pip install -r requirements.txt
# copy project
COPY . /usr/src/app
EXPOSE 8000
CMD ["python", "manage.py", "runserver", "0.0.0.0:8000"]