docker: debian/s390x no more in unstable, now available in Stretch

docker-image-debian-armhf-cross: docker-image-debian9

docker-image-debian-arm64-cross: docker-image-debian9

docker-image-debian-mipsel-cross: docker-image-debian9

docker-image-debian-s390x-cross: docker-image-debian9

# Expand all the pre-requistes for each docker image and test combination

$(foreach i,$(DOCKER_IMAGES), \

#

# Docker s390 cross-compiler target

#

# This docker target is based on stretch (testing) as the stable build

# doesn't have the cross compiler available.

# This docker target builds on the debian Stretch base image.

#

FROM debian:testing-slim

# Duplicate deb line as deb-src

RUN cat /etc/apt/sources.list | sed "s/deb/deb-src/" >> /etc/apt/sources.list

FROM qemu:debian9

# Add the s390x architecture

RUN dpkg --add-architecture s390x

# Grab the updated list of packages

RUN apt update && apt dist-upgrade -yy

RUN apt install -yy build-essential clang

RUN apt-get build-dep -yy -a s390x qemu || apt-get -f install

RUN apt install -yy gcc-multilib-s390x-linux-gnu binutils-multiarch

RUN DEBIAN_FRONTEND=noninteractive eatmydata \

    apt-get install -y --no-install-recommends \

        gcc-multilib-s390x-linux-gnu

RUN DEBIAN_FRONTEND=noninteractive eatmydata \

    apt-get build-dep -yy -a s390x qemu

# Specify the cross prefix for this image (see tests/docker/common.rc)

ENV QEMU_CONFIGURE_OPTS --cross-prefix=s390x-linux-gnu-

RUN DEBIAN_FRONTEND=noninteractive \

# Install extra libraries to increase code coverage

RUN DEBIAN_FRONTEND=noninteractive eatmydata \

    apt-get install -y --no-install-recommends \

        glusterfs-common:s390x \

        libbz2-dev:s390x \