Introduce immutable repos (#64)

* Introduce immutable repos

* Update error message to use the one in master

_data/toc.yaml

@@ -1830,6 +1830,8 @@ manuals:
           title: Delete images
         - path: /datacenter/dtr/2.3/guides/user/manage-images/scan-images-for-vulnerabilities/
           title: Scan images for vulnerabilities
+        - path: /datacenter/dtr/2.3/guides/user/manage-images/prevent-tags-from-being-overwritten/
+          title: Prevent tags from being overwritten
         - sectiontitle: Sign images
           section:
           - path: /datacenter/dtr/2.3/guides/user/manage-images/sign-images/

datacenter/dtr/2.3/guides/user/manage-images/prevent-tags-from-being-overwritten.md

@@ -0,0 +1,36 @@
+---
+title: Prevent tags from being overwritten
+description: Learn how to make your tags immutable and don't allow users to
+  overwrite a tag if it already exists
+keywords: docker, registry, immutable
+---
+
+By default, users with access to push to a repository, can push the same tag
+multiple times to the same repository.
+As an example, a user pushes an image to `site/wordpress:4.7`, and later another
+user can push the image with exactly the same name but different functionality.
+This might make it difficult to trace back the image to the build that generated
+it.
+
+To prevent this from happening you can configure a repository to be immutable.
+Once you push a tag, DTR won't anyone else to push another tag with the same
+name.
+
+## Make tags immutable
+
+To make tags immutable, in the **DTR web UI**, navigate to the
+**repository settings** page, and change **immutability** to **on**.
+
+![](../../images/immutable-repo-1.png)
+
+From now on, users will get an error message when trying to push a tag
+that already exists:
+
+```none
+docker push dtr.example.org/site/wordpress:4.7
+unknown: tag=4.7 cannot be overwritten because dtr.example.org/site/wordpress is an immutable repository
+```
+
+## Where to go next
+
+* [Sign images](sign-images/index.md)