why installing pandas take time in docker

FROM python:3.8-alpine
RUN echo "@testing http://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories
RUN apk add --update --no-cache py3-numpy py3-pandas@testing
ENV PYTHONPATH=/usr/lib/python3.8/site-packages

COPY . /app
WORKDIR /app

RUN pip install -r requirements.txt

EXPOSE 5003 
ENTRYPOINT [ "python" ] 
CMD [ "app.py" ]