Add Debian Buster Stein backports APT repository

Change-Id: Ie140cf0d6308a5ee3c09f06ddfcf63be2e1cec64
(manual picked from commit c08a4b5)

Author: viktormichalek

docker/base/Dockerfile.j2

@@ -353,7 +353,8 @@ COPY apt_preferences.{{ base_distro }} /etc/apt/preferences
     {% set remote_apt_keys = [
       'http://obs.linaro.org/ERP:/18.06/Debian_9/Release.key',
       'https://download.docker.com/linux/debian/gpg',
-      'https://packages.grafana.com/gpg.key'
+      'https://packages.grafana.com/gpg.key',
+      'http://buster-stein.debian.net/debian/dists/pubkey.gpg',
     ] %}
     {% set base_apt_packages = base_apt_packages +
       ['sudo',]

docker/base/sources.list.debian

@@ -11,3 +11,7 @@ deb [arch=amd64] https://artifacts.elastic.co/packages/5.x/apt stable main
 
 # main docker repo
 deb https://download.docker.com/linux/debian buster stable
+
+# Buster - Stein repos
+deb http://buster-stein.debian.net/debian buster-stein-backports main
+deb http://buster-stein.debian.net/debian buster-stein-backports-nochange main