Try using cache instead of local registry

We don't need a server and can just attempt to save the image to later load it.

.gitlab-ci.yml

 image: docker:19.03.12
 
 stages:
-  - docker_setup
   - docker_test
   - docker_push
 
@@ -24,55 +23,47 @@ variables:
   # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
   DOCKER_HOST: tcp://docker:2376
 
-  # Support caching images between jobs
-  CI_LOCAL_REGISTRY_NAME: "localhost:5000"
-  CI_LOCAL_REGISTRY_IMAGE: "localhost:5000/reseed-tools"
+.docker_cache:
+  cache:
+    key: "$CI_COMMIT_REF_SLUG"
+    paths:
+      - exports/*.tar
 
 services:
-  - name: docker:19.03.12-dind
-    command:
-      - --insecure-registry
-      - localhost:5000
+  - docker:19.03.12-dind
 
 # Login to job registry and publish registry
 before_script:
   - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
 
-start_registry:
-  stage: docker_setup
-  script:
-    - docker run -d -p 5000:5000 --name registry registry:2
-    - docker ps -a
-
 # Make sure we can build a docker image
 # It's pushed to the pipeline-local registry which acts as a cache
 build_docker:
+  extends:
+    - .docker_cache
   stage: docker_test
   script:
-    - docker ps -a
-    - docker ps -a | grep registry
-    - docker pull $CI_REGISTRY_IMAGE:latest || true
-    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_LOCAL_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA --tag $CI_LOCAL_REGISTRY_IMAGE:latest .
-    - docker push $CI_LOCAL_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA
-    - docker push $CI_LOCAL_REGISTRY_IMAGE:latest
+    # Try to load latest branch image from local tar or from registry
+    - docker load exports/$CI_COMMIT_REF_SLUG.tar || docker pull $CI_REGISTRY_IMAGE:latest || true
+    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:latest .
+    - docker save $CI_REGISTRY_IMAGE:latest > exports/$CI_COMMIT_REF_SLUG.tar
 
 test_pull_local_docker:
+  extends:
+    - .docker_cache
   stage: docker_push
   script:
-    - docker pull $CI_LOCAL_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA
+    - docker load exports/$CI_COMMIT_REF_SLUG.tar
 
 push_docker:
   stage: docker_push
+  extends:
+    - .docker_cache
   script:
-    - docker pull $CI_LOCAL_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA || true
-    - docker tag $CI_LOCAL_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
-    - docker tag $CI_LOCAL_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA $CI_REGISTRY_IMAGE:latest
+    - docker load exports/$CI_COMMIT_REF_SLUG.tar
+    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG --tag $CI_REGISTRY_IMAGE:latest .
     - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
     - docker push $CI_REGISTRY_IMAGE:latest
-#    - docker pull $CI_REGISTRY_IMAGE:latest || true
-#    - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG --tag $CI_REGISTRY_IMAGE:latest .
-#    - docker push $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
-#    - docker push $CI_REGISTRY_IMAGE:latest
   only:
     - master
     - tags