[ci] tag Docker images built from the master branch

and use them as astarting point for docker builds on MRs/branches, which should speed up those builds in most cases

.gitlab-ci.yml

@@ -115,17 +115,16 @@ test:docker:
 
 
 .docker:tag: &docker_tag
-    stage: deploy
-    image: docker:18
+    stage: "deploy"
+    image: "docker:${DOCKER_VERSION}"
     before_script:
         - docker info
     script:
         - docker tag ${IMAGE_REGISTRY}:${CI_COMMIT_SHA} ${REGISTRY_IMAGE}:${TAG_NAME}
 
 
-deploy:docker-master: *docker_tag
-    stage: "deploy"
-    image: "docker:${DOCKER_VERSION}"
+deploy:docker-master:
+    <<: *docker_tag
     only: "master"
     variables:
         TAG_NAME: "master"