[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

 stages:
     - "docker"
     - "test"
+    - "deploy"
     - "cleanup"
 
 variables:
     DOCKER_VERSION: 18.09.9
     IMAGE_REGISTRY: renewal/recsystems-baseline
+    TEMP_IMAGE_TAG: ${CI_COMMIT_SHA}-${CI_BUILD_ID}
 
 # Override the default workflow rules to only run pipelines on the master
 # branch, merge requests, or branches that contain the string 'ci/'
@@ -29,7 +31,9 @@ docker:build:
     before_script:
         - "docker info"
     script:
-        - "docker build --tag ${IMAGE_REGISTRY}:${CI_COMMIT_SHA} ."
+        - >
+            docker build --cache-from ${IMAGE_REGISTRY}:master
+            --tag ${IMAGE_REGISTRY}:${TEMP_IMAGE_TAG} .
 
 
 .test: &test
@@ -118,13 +122,30 @@ test:docker:
     artifacts: {}
 
 
+.docker:tag: &docker_tag
+    stage: "deploy"
+    image: "docker:${DOCKER_VERSION}"
+    before_script:
+        - docker info
+    script:
+        - docker tag ${IMAGE_REGISTRY}:${TEMP_IMAGE_TAG} ${REGISTRY_IMAGE}:${TAG_NAME}
+
+
+deploy:docker-master:
+    <<: *docker_tag
+    only:
+        - "master"
+    variables:
+        TAG_NAME: "master"
+
+
 cleanup:docker:
     stage: "cleanup"
-    image: "docker:18"
+    image: "docker:${DOCKER_VERSION}"
     only: *only
     when: "always"
     before_script:
         - docker info
     script:
-        - docker rmi ${IMAGE_REGISTRY}:${CI_COMMIT_SHA}
+        - docker rmi ${IMAGE_REGISTRY}:${TEMP_IMAGE_TAG}
         - docker rmi $(docker images --filter dangling=true -q --no-trunc ${IMAGE_REGISTRY}) || true