That extra request to the default /favicon.ico path on the root of the
domain, which is not a valid path because Pages has no root namespace,
was being redirected to an auth page. That auth page was setting cookies
which were then being passed along with subsequent requests, causing the
pages to not be cached. Hopefully this fixes that.

* custom-domain:
  Re-enable Cloudflare purge, but only on default branch since pages only runs on default branch
  Go back to not using depend strategy on triggered pipeline
  Purposefully fail the child job to see if the parent job fails too
  Set the trigger strategy to 'depend' and see if it reserves a CI runner
  Move the cache purge to a child pipeline
  Delay the Cloudflare cache purge so that pages:deploy is finished
  Add a subdirectory and page for testing purposes
  Purge all site URLs from Cloudflare cache
  Add a cache-test.html page for testing page cache
  Temp change to make pages run on this branch

My tests indicate that the pages:deploy job typically takes 50-60
seconds to complete, so I'm delaying the cache purge for 180 seconds

Variable substitution

See merge request gitlab-tests/pages-test!5