CI: optimize packaging tests

Close #541 (closed).

ci/debian-buster/Dockerfile

@@ -65,6 +65,9 @@ RUN pip3 install flake8
 # Python requests for CI scripts
 RUN pip3 install requests
 
+# docker-py for packaging tests
+RUN pip3 install docker
+
 # Unbound for respdiff
 RUN apt-get install unbound unbound-anchor -y -qqq
 RUN printf "server:\n interface: 127.0.0.1@53535\n use-syslog: yes\n do-ip6: no\nremote-control:\n control-enable: no\n" >> /etc/unbound/unbound.conf