Upgrade yarn to 1.12 in build image of gitlab:assets:compile job

.gitlab-ci.yml

@@ -638,7 +638,7 @@ gitlab:setup-mysql:
 # Frontend-related jobs
 gitlab:assets:compile:
   <<: *dedicated-no-docs-and-no-qa-pull-cache-job
-  image: dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.5.3-git-2.18-chrome-69.0-node-8.x-yarn-1.2-graphicsmagick-1.3.29-docker-18.06.1
+  image: dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.5.3-git-2.18-chrome-69.0-node-8.x-yarn-1.12-graphicsmagick-1.3.29-docker-18.06.1
   dependencies: []
   services:
     - docker:stable-dind