mariadb: install mariabackup on Debian

On AArch64 systems we are unable to deploy Stein due to lack of
mariabackup command.

Change-Id: I00b8c945470a0f63f23f2d0da74eed3189f5d9e5
Closes-bug: #1867750

docker/mariadb/Dockerfile.j2

@@ -27,6 +27,7 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
     ] %}
     {% if base_distro == 'debian' %}
         {% set mariadb_packages = mariadb_packages + [
+            'mariadb-backup',
             'mariadb-server'
         ] %}
     {% else %}