gitlab-ci: try to use smaller images and avoid install recommends

gitlab-ci/docker/jessie-backports/Dockerfile

@@ -4,11 +4,11 @@ RUN apt-get update
 RUN apt-get upgrade -y
 
 # Minimal tools required by acme-dns-tiny CI
-RUN apt-get install -y \
+RUN apt-get install -y --no-install-recommends \
 	python3-coverage \
 	python3-pip
 
-RUN apt-get install -y \
+RUN apt-get install -y --no-install-recommends \
 	-t jessie-backports \
 	python3-configargparse \
 	python3-dnspython

gitlab-ci/docker/jessie/Dockerfile

-FROM debian:jessie
+FROM debian:jessie-slim
 
 RUN apt-get update
 RUN apt-get upgrade -y
 
 # Minimal tools required by acme-dns-tiny CI
-RUN apt-get install -y \
+RUN apt-get install -y --no-install-recommends \
 	python3-dnspython \
 	python3-coverage \
 	python3-pip

gitlab-ci/docker/stretch/Dockerfile

-FROM debian:stretch
+FROM debian:stretch-slim
 
 RUN apt-get update
 RUN apt-get upgrade -y
 
 # Minimal tools required by acme-dns-tiny CI
-RUN apt-get install -y \
+RUN apt-get install -y --no-install-recommends \
 	python3-dnspython \
 	python3-coverage \
 	python3-configargparse \