Failed to install openjdk in Debian Stretch

My Dockerfile is:

FROM python:3.7-slim-stretch

RUN echo "deb http://security.debian.org/debian-security  stretch/updates main" >> /etc/apt/sources.list

RUN apt-get update && apt-get install \
    --no-install-recommends -qq -y \
    apt-utils \
    poppler-utils \
    build-essential \
    cmake \
    libfreetype6-dev \
    libfontconfig-dev \
    libjpeg-dev \
    libopenjp2-7-dev \
    libcairo2-dev \
    pdftk \
    ghostscript \
    autoconf \
    automake \
    libtool \
    autoconf-archive \
    pkg-config \
    libpng-dev \
    libjpeg62-turbo-dev \
    libtiff5-dev \
    zlib1g-dev \
    imagemagick \
    wget \
    procps \
    curl \
    locales \
    libicu-dev \
    libpango1.0-dev \
    libcairo2-dev \
    g++ \
    make

RUN mkdir -p /usr/share/man/man1 && apt install -y openjdk-8-jdk


ENTRYPOINT [ "/bin/bash", "-l" ]

But it started to fail during installation of openjdk-8:

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
The command '/bin/sh -c mkdir -p /usr/share/man/man1 && apt install -y openjdk-8-jdk' returned a non-zero code: 100

I've got it to work adding stretch-backports to sources list, but I don't know why it works. As far as I know, backports are disabled by default:

RUN echo "deb http://security.debian.org/debian-security  stretch/updates main" >> /etc/apt/sources.list
RUN echo "deb http://deb.debian.org/debian  stretch-backports main" >> /etc/apt/sources.list

I would like to know why it starts to break only now and why adding stretch-backports or http://security.debian.org/debian-security oldoldstable/updates main to sources make it works.