Add troubleshooting step for older docker clients

* The container registry needs a compatibility setting enabled for older
docker clients. This will document how to enable the setting for
omnibus and source installations.

doc/administration/container_registry.md

@@ -659,6 +659,37 @@ Start with a value between `25000000` (25MB) and `50000000` (50MB).
 
 1. Save the file and [restart GitLab][] for the changes to take effect.
 
+### Supporting older Docker clients
+
+As of GitLab 11.9, we began shipping version 2.7.1 of the Docker container registry, which disables the schema1 manifest by default. If you are still using older Docker clients (1.9 or older), you may experience an error pushing images. See [omnibus-4145](https://gitlab.com/gitlab-org/omnibus-gitlab/issues/4145) for more details.
+
+You can add a configuration option for backwards compatibility.
+
+**For Omnibus installations**
+
+1. Edit `/etc/gitlab/gitlab.rb`:
+
+    ```ruby
+    registry['compatibility_schema1_enabled'] = true
+    ```
+
+1. Save the file and [reconfigure GitLab][] for the changes to take effect.
+
+---
+
+**For installations from source**
+
+1. Edit the YML configuration file you created when you [deployed the registry][registry-deploy]. Add the following snippet:
+
+    ```yaml
+    compatibility:
+        schema1:
+            enabled: true
+    ```
+
+1. Restart the registry for the changes to take affect.
+
+
 [ce-18239]: https://gitlab.com/gitlab-org/gitlab-ce/issues/18239
 [docker-insecure-self-signed]: https://docs.docker.com/registry/insecure/#use-self-signed-certificates
 [reconfigure gitlab]: restart_gitlab.md#omnibus-gitlab-reconfigure