Update to use gitlab-ci-helpers

.gitlab-ci.yml

-image: registry.uninett.no/system/docker-builder:1.13.1-uninett7
+include:
+  - project: 'asm/gitlab-ci-helpers'
+    file: '/gitlab-ci-helpers.yml'
+
+stages:
+  - build
 
 development:
-  stage: deploy
-  script:
-    - docker build -t "registry.uninett.no/$CI_PROJECT_PATH:latest" .
-    - docker push "registry.uninett.no/$CI_PROJECT_PATH:latest"
+  extends: .docker-build
+  stage: build
+  variables:
+    CI_REGISTRY_TAG: latest
   only:
     - master
 
 stable:
-  stage: deploy
-  script:
-    - docker build -t "registry.uninett.no/$CI_PROJECT_PATH:$CI_BUILD_REF_NAME" .
-    - docker push "registry.uninett.no/$CI_PROJECT_PATH:$CI_BUILD_REF_NAME"
+  extends: .docker-build
+  stage: build
+  variables:
+    CI_REGISTRY_TAG: $CI_COMMIT_REF_NAME
   only:
     - tags