Fix NGINX not routing traffic to the right server

What does this MR do?

Prior to !8687 (merged), attempting to access GitLab with a different hostname (e.g. localhost) worked because NGINX picks the first configuration file loaded if server_name does not match. Because gitlab-http.conf preceded gitlab-kas.conf and gitlab-pages.conf, by default requests routed to the Rails server.

However, the refactor renamed gitlab-http.conf to gitlab-rails.conf, which caused NGINX to route traffic with a non-matching server_name to KAS or Pages. For example, if Pages were enabled, curl http://localhost/-/health routes to the Pages server instead of Rails.

Fix this by restoring the original, implicit behavior by marking the Rails server as default_server in the NGINX conf.

Related issues

Relates to:

• #9403 (closed)
• #9406 (closed)

How to validate locally

1. In /etc/gitlab/gitlab.rb:

pages_external_url "http://pages.example.com/"
gitlab_pages['enable'] = true

2. Run gitlab-ctl reconfigure

3. Run curl -v http://localhost/-/health. You should see GitLab OK instead of a 404.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.