Merge branch 'docs-dockerfile-templates' into 'master'

Improve docs for Dockerfile templates API

See merge request gitlab-org/gitlab!56502

doc/api/templates/dockerfiles.md

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 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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 # Dockerfiles API
 
-In GitLab, there is an API endpoint available for Dockerfiles. For more
-information on Dockerfiles, see the
-[Docker documentation](https://docs.docker.com/engine/reference/builder/).
+GitLab provides an API endpoint for instance-level Dockerfile templates.
+Default templates are defined at
+[`vendor/Dockerfile`](https://gitlab.com/gitlab-org/gitlab-foss/-/tree/master/vendor/Dockerfile)
+in the GitLab repository.
+
+## Override Dockerfile API templates **(PREMIUM SELF)**
+
+In [GitLab Premium and higher](https://about.gitlab.com/pricing) tiers, GitLab instance
+administrators can override templates in the
+[Admin Area](../../user/admin_area/settings/instance_template_repository.md).
 
 ## List Dockerfile templates
 

doc/user/admin_area/settings/index.md

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