Docker: Install xz-utils from stretch

The stretch version supports multi-thread compression, allowing to
accelerate e.g. artifact compression tasks. The newer version of xz also
seem to resolve mysterious compression failures that created too short
output images.

As we install the stretch source in the base image now, we can remove
that port from the isar image and align both regarding the setup.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -16,6 +16,10 @@ RUN apt-get update && \
                        syslinux tree python3-pip bc python3-yaml \
                        lsb-release python3-setuptools ssh-client \
                        vim less mercurial && \
+    echo 'deb http://deb.debian.org/debian stretch main' >> /etc/apt/sources.list.d/backports.list && \
+    apt-get update && \
+    apt-get install -y -f --no-install-recommends --target-release stretch \
+            xz-utils && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*