Merge branch 'leipert-update-node' into 'master'

Update node and yarn to match what is in our docs

See merge request gitlab-org/build/CNG!234

gitlab-rails/Dockerfile.erb

@@ -22,8 +22,8 @@ ARG DATADIR=/var/opt/gitlab
 ARG CPU_COUNT=4
 ARG GITLAB_USER=git
 ARG COMPILE_ASSETS_ENV="RAILS_ENV=production NODE_ENV=production USE_DB=false SKIP_STORAGE_VALIDATION=true"
-ARG NODE_VERSION=8.12.0
-ARG YARN_VERSION=1.10.1
+ARG NODE_VERSION=12.4.0
+ARG YARN_VERSION=1.16.0
 
 
 # install runtime deps