GitLab

- Sets `BUILD_ASSETS_IMAGE` to `true` in `.gitlab-ci.yml` (previously
  set to false and overridden to true in the project's variable).
  - There's no need to set it to `false` by default and override it to
    `true` in the project's variable; instead, we can just default to
    `true` in `.gitlab-ci.yml` and override it to `false` in the
    project's variable if needed.
- Changes implementation of `Trigger::CNG#trigger_token` to
  `ENV['BUILD_TRIGGER_TOKEN'] || ENV['CI_JOB_TOKEN']`.
  `ENV['CI_JOB_TOKEN']` will be used used for `cloud-native-image`,
  `ENV['BUILD_TRIGGER_TOKEN']` is set by `review-build-cng`.
- Changes `cache-repo` to cache a shallow clone (and no full clone) by
  default. If the `DISABLE_SHALLOW_REPO_CACHING` variable is set,
  shallow clone caching will be disabled, and full clone caching will
  be enabled instead.
- Sets `DOCS_GITLAB_REPO_SUFFIX` to `ee` in `.gitlab-ci.yml`
  (previously set as a project's variable).
- Sets `REVIEW...