docs: Adding a private OCI registry section

Signed-off-by: GuillaumeAssier <[email protected]>

Author: GuillaumeASSIER

docs/src/registries-and-offline.md

@@ -21,6 +21,14 @@ insecure=true
 
 For more, see [containers-registries.conf](https://github.com/containers/image/blob/main/docs/containers-registries.conf.5.md).
 
+## Private registries
+
+It's common to use a private repository when deploying a fleet of bootc instances.
+
+In addition to registry configuration, if you want to use a private registry, you'll need to address authentication. Place a `auth.json` file in the directory `/etc/ostree/auth.json`.
+
+For more, see [auth.json](https://man.archlinux.org/man/containers-auth.json.5)
+
 ## Disconnected and offline updates
 
 It is common (a best practice even) to maintain systems which default