[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

@@ -7,6 +7,7 @@ stages:
 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/'
@@ -32,7 +33,7 @@ docker:build:
     script:
         - >
             docker build --cache-from ${IMAGE_REGISTRY}:master
-            --tag ${IMAGE_REGISTRY}:${CI_COMMIT_SHA} .
+            --tag ${IMAGE_REGISTRY}:${TEMP_IMAGE_TAG} .
 
 
 .test: &test
@@ -115,18 +116,18 @@ 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}
+        - docker tag ${IMAGE_REGISTRY}:${TEMP_IMAGE_TAG} ${REGISTRY_IMAGE}:${TAG_NAME}
 
 
-deploy:docker-master: *docker_tag
-    stage: "deploy"
-    image: "docker:${DOCKER_VERSION}"
-    only: "master"
+deploy:docker-master:
+    <<: *docker_tag
+    only:
+        - "master"
     variables:
         TAG_NAME: "master"
 
@@ -139,5 +140,5 @@ cleanup:docker:
     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