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

@@ -1,25 +1,22 @@
-# This image builds Yocto 2.1 and 2.2 jobs using the kas tool
+# This image builds Yocto jobs using the kas tool
 
-FROM debian:jessie-slim
+FROM debian:stretch-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
 
-ENV LOCALE=en_US.UTF-8
 RUN apt-get update && \
-    apt-get install --no-install-recommends -y locales && \
-    sed -i -e "s/# $LOCALE/$LOCALE/" /etc/locale.gen && \
-    ln -s /etc/locale.alias /usr/share/locale/locale.alias && \
-    dpkg-reconfigure --frontend=noninteractive locales && \
-    apt-get install --no-install-recommends -y \
+    apt-get install -y locales && \
+    localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
+ENV LANG=en_US.utf8
+
+RUN apt-get install --no-install-recommends -y \
                        gawk wget git-core diffstat unzip file \
                        texinfo gcc-multilib build-essential \
                        chrpath socat cpio python python3 rsync \
                        tar bzip2 curl dosfstools mtools parted \
                        syslinux tree python3-pip bc python3-yaml \
                        lsb-release python3-setuptools ssh-client \
-                       vim less mercurial iproute2 && \
-    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 && \
+                       vim less mercurial iproute2 xz-utils && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
@@ -34,6 +31,4 @@ ENV NO_PROXY="*"
 COPY . /kas
 RUN pip3 --proxy=$https_proxy install /kas
 
-ENV LANG=$LOCALE
-
 ENTRYPOINT ["/kas/docker-entrypoint"]