WIP: Use high CPU runners for assets compilation jobs

What does this MR do?

Experiment to use high CPU runners for assets compilation jobs, suggested in https://gitlab.com/gitlab-org/gitlab-ce/issues/56538#note_192485436.

That looks promising:

Job	master	this MR
compile-assets rake:assets:precompile	82613ms, 82945ms, 82281ms	65527ms
compile-assets total	11m 10s, 8m 39s, 8m 45s	6m 26s
gitlab:assets:compile rake:assets:precompile	479239ms, 520504ms, 588495ms	500880ms
gitlab:assets:compile	19m 3s, 19m 34s, 21m 13s	17m 7s

.gitlab/ci/frontend.gitlab-ci.yml

@@ -18,6 +18,7 @@
   image: dev.gitlab.org:5005/gitlab/gitlab-build-images:ruby-2.6.3-git-2.21-chrome-73.0-node-12.x-yarn-1.16-graphicsmagick-1.3.29-docker-18.06.1
   tags:
     - gitlab-org
+    - docker
     - high-cpu
   dependencies:
     - setup-test-env
@@ -56,9 +57,6 @@
     - /.+/@gitlab-org/gitlab-ee
     - /.+/@gitlab/gitlabhq
     - /.+/@gitlab/gitlab-ee
-  tags:
-    - docker
-    - gitlab-org
 
 gitlab:assets:compile:
   extends: .gitlab:assets:compile-metadata