Docker: Update to Debian stretch

Time to move forward, buster is almost in sight. We may break some older
Yocto versions this way, but they can always use older images.

Along this update, this migrates the locale setup towards the
recommendation on https://docs.docker.com/samples/library/debian/#locales.
Furthermore, DEBIAN_FRONTEND=noninteractive is moved to the top-level
Dockerfile - it's generic.

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

Dockerfile.isar

@@ -1,17 +1,13 @@
+# This image builds Isar jobs using the kas tool
+
 FROM kasproject/kas:latest
 
-ENV DEBIAN_FRONTEND noninteractive
-
 ENV LC_ALL=en_US.UTF-8
-RUN echo 'deb http://deb.debian.org/debian jessie-backports main' >> /etc/apt/sources.list.d/backports.list && \
-    apt-get update && \
-    apt-get install -y -f --no-install-recommends --target-release jessie \
+RUN apt-get update && \
+    apt-get install -y -f --no-install-recommends \
             autoconf automake gdisk libtool bash-completion \
             sudo grub2 grub-efi-amd64-bin grub-efi-ia32-bin \
-            reprepro python3 binfmt-support && \
-    apt-get install -y -f --no-install-recommends --target-release jessie-backports \
-            e2fsprogs && \
-    apt-get install -y -f --no-install-recommends --target-release stretch \
+            reprepro python3 binfmt-support e2fsprogs \
             multistrap qemu-user-static debootstrap && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp*