Revert "Docker: move on to buster"

This reverts commit 97942197bc.

This is only temporarily in order to release another 1.x version that is
based on stretch. Older Yocto versions, maybe up to 2.4 or 2.5, require
stretch due to incompatibilities with host tools such as git.

After that release, this commit will be restored, and we will roll out a
2.x series that shall use buster in its images.

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

Dockerfile

@@ -1,6 +1,6 @@
 # This image builds Yocto jobs using the kas tool
 
-FROM debian:buster-slim
+FROM debian:stretch-slim
 
 ENV DEBIAN_FRONTEND=noninteractive
 

Dockerfile.isar

@@ -3,7 +3,9 @@
 FROM kasproject/kas:latest
 
 ENV LC_ALL=en_US.UTF-8
-RUN apt-get update && \
+RUN echo 'deb http://deb.debian.org/debian buster main' > /etc/apt/sources.list.d/buster.list && \
+    echo "Package: qemu-user-static binfmt-support\nPin: release n=buster\nPin-Priority: 501\n\nPackage: *\nPin: release n=buster\nPin-Priority: -1" > /etc/apt/preferences.d/qemu-user-static && \
+    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 \