Fix Docker Debian Jessie issues. Dockerfile changes to get around the apt-get issue in Debian Jessie. Shamelessly copied and adapted from discussion on Stackoverflow here https://unix.stackexchange.com/questions/508724/failed-to-fetch-jessie-backports-repository

Dockerfile

FROM python:3.6-slim-jessie
MAINTAINER Raveen Beemsingh <raveenb@hammerhead.io>

RUN apt-get update && \
    apt-get -y install cron curl redis-server sudo build-essential && \
    rm -rf /var/lib/apt/lists/*

RUN adduser appuser
RUN echo "appuser ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers

WORKDIR /home/appuser
COPY requirements.txt requirements.txt

RUN python -m venv venv

# jessie issue
RUN echo "deb [check-valid-until=no] http://cdn-fastly.deb.debian.org/debian jessie main" > /etc/apt/sources.list.d/jessie.list
RUN echo "deb [check-valid-until=no] http://archive.debian.org/debian jessie-backports main" > /etc/apt/sources.list.d/jessie-backports.list
RUN sed -i '/deb http:\/\/deb.debian.org\/debian jessie-updates main/d' /etc/apt/sources.list
RUN apt-get -o Acquire::Check-Valid-Until=false update

# couchbase specific installations
RUN apt-get -y install lsb-release wget && \
    rm -rf /var/lib/apt/lists/*

RUN lsb_release -a
RUN wget http://packages.couchbase.com/ubuntu/couchbase.key
RUN apt-key add couchbase.key
RUN touch /etc/apt/sources.list.d/couchbase.list
RUN echo "deb http://packages.couchbase.com/ubuntu stretch stretch/main" > /etc/apt/sources.list.d/couchbase.list
RUN wget http://packages.couchbase.com/releases/couchbase-release/couchbase-release-1.0-4-amd64.deb
RUN dpkg -i couchbase-release-1.0-4-amd64.deb

RUN apt-get -o Acquire::Check-Valid-Until=false update && \
    apt-get -y install libcouchbase-dev && \
    rm -rf /var/lib/apt/lists/*

RUN rm couchbase-release-1.0-4-amd64.deb couchbase.key

RUN venv/bin/pip install -r requirements.txt

COPY app app
COPY .env app.py boot.sh config.py cronjob tasks.py task_settings.py ./

RUN chmod +x boot.sh

ENV FLASK_APP app.py

RUN chown -R appuser:appuser ./
USER appuser

EXPOSE 5000
ENTRYPOINT ["./boot.sh"]

https://unix.stackexchange.com/questions/508724/failed-to-fetch-jessie-backports-repository