Docs for [Geo] Docker Registry replication

changelogs/unreleased-ee/docker_registry_geo.yml

+---
+title: Geo - add documentation about using shared a S3 bucket with GitLab Container Registry
+merge_request:
+author:
+type: other

doc/gitlab-geo/README.md

@@ -190,14 +190,18 @@ Read through the [Geo High Availability documentation](ha.md).
 When you have object storage enabled, please consult the
 [Geo with Object Storage](object_storage.md) documentation.
 
+## Replicating the Container Registry
+
+Read how to [replicate the Container Registry](docker_registry.md).
+
 ## Current limitations
 
 - You cannot push code to secondary nodes
 - The primary node has to be online for OAuth login to happen (existing
   sessions and Git are not affected)
 - It works for repos, wikis, issues, and merge requests
-- It does not work for job logs, artifacts, Docker images of the Container
-  Registry, and GitLab Pages
+- It does not work for job logs, artifacts, GitLab Pages, and Docker images of the Container
+  Registry (by default, but you can configure it separately, see [replicate the Container Registry](docker_registry.md) for details)
 - It does not work for attachments uploaded before GitLab 9.0 because they will
   not present in the uploads table, until
   [#29249](https://gitlab.com/gitlab-org/gitlab-ce/issues/29240) is fixed.

doc/gitlab-geo/docker_registry.md

+# Docker Registry for a secondary node
+
+You can setup a [Docker Registry](https://docs.docker.com/registry/) on your
+secondary Geo node that mirrors the one on the primary Geo node.
+
+## Storage support
+
+CAUTION: **Warning:**
+If you use [local storage](../administration/container_registry.md#container-registry-storage-driver)
+for the Container Registry you **cannot** replicate it to the secondary Geo node.
+
+Docker Registry currently supports a few types of storages. If you choose a
+distributed storage (`azure`, `gcs`, `s3`, `swift`, or `oss`) for your Docker
+Registry on a primary Geo node, you can use the same storage for a secondary
+Docker Registry as well. For more information, read the
+[Load balancing considerations](https://docs.docker.com/registry/deploying/#load-balancing-considerations)
+when deploying the Registry, and how to setup the storage driver for GitLab's
+integrated [Container Registry](../administration/container_registry.md#container-registry-storage-driver).
+
+[ee]: https://about.gitlab.com/gitlab-ee/

doc/gitlab-geo/faq.md

@@ -34,3 +34,7 @@ connectivity between your nodes, your hardware, etc.
 We send the clone url from the primary server to any secondaries, so it
 doesn't matter. If primary is running on port `2200`, clone url will reflect
 that.
+
+## Is this possible to set up a Docker Registry for a secondary node that mirrors the one on a primary node?
+
+Yes. See [Docker Registry for a secondary Geo node](docker_registry.md).