gitlab-ci: update dockerfile and use tags to use runner with good image

gitlab-ci/docker/jessie/Dockerfile

 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 --no-install-recommends \
-	python3-dnspython \
-	python3-coverage \
-	python3-pip
+RUN apt-get update \
+	&& apt-get install -y --no-install-recommends \
+		python3-dnspython \
+		python3-coverage \
+		python3-pip \
+	&& apt-get clean
 
 # Allows run python3-coverage with same command than manual install by pip
 RUN update-alternatives --install \

gitlab-ci/docker/stretch/Dockerfile

 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 --no-install-recommends \
-	python3-dnspython \
-	python3-coverage \
-	python3-configargparse \
-	python3-pip
+RUN apt-get update \
+	&& apt-get install -y --no-install-recommends \
+		python3-dnspython \
+		python3-coverage \
+		python3-configargparse \
+		python3-pip
 
 # Allows run python3-coverage with same command than manual install by pip
 RUN update-alternatives --install \

gitlab-ci/gitlab-ci.yml

 jessie:
-  image: adt-jessie
+  tags:
+    - jessie
   before_script:
     - pip3 install --upgrade -r tests/requirements.txt
   script:
@@ -8,7 +9,8 @@ jessie:
     - coverage html
 
 stretch:
-  image: adt-stretch
+  tags:
+    - stretch
   before_script:
     - pip3 install --upgrade -r tests/requirements.txt
   script: