[ci] tag Docker images built from the master branch, and use them as a

starting 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:
@@ -29,7 +30,9 @@ docker:build:
     before_script:
         - "docker info"
     script:
-        - "docker build --tag ${IMAGE_REGISTRY}:${CI_COMMIT_SHA} ."
+        - >
+            docker build --cache-from ${IMAGE_REGISTRY}:master
+            --tag ${IMAGE_REGISTRY}:${CI_COMMIT_SHA} .
 
 
 .test: &test
@@ -111,9 +114,26 @@ test:docker:
     artifacts: {}
 
 
+.docker:tag: &docker_tag
+    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
+    only:
+        - "master"
+    variables:
+        TAG_NAME: "master"
+
+
 cleanup:docker:
     stage: "cleanup"
-    image: "docker:18"
+    image: "docker:${DOCKER_VERSION}"
     only: *only
     when: "always"
     before_script: