Add documentation for feature in plone#92

https://github.com/plone/plone-backend/pull/92/files related.

Author: Rudd-O

docs/install/containers/images/backend.md

@@ -65,6 +65,13 @@ Plone 6 example:
 docker run -p 8081:8081 -e LISTEN_PORT=8081 plone/plone-backend:{PLONE_BACKEND_VERSION}
 ```
 
+#### Adding configuration to `zope.conf`
+
+It is not possible to add configuration fragments to `zope.conf` directly, like
+it is with the `buildout` deployment method.  However, you can derive your own
+container image, and drop in configuration fragments.  See *Extending from this
+image* below for instructions.
+
 ### Site creation variables
 
 | Environment variable | Description |
@@ -304,6 +311,19 @@ And start a container.
 docker run -p 8080:8080 myproject:latest start
 ```
 
+### Adding `zope.conf` configuration fragments
+
+In the directory containing your `Dockerfile`, create a folder `etc/zope.conf.d`.
+Add your `zope.conf` configuration fragments there.
+
+Now add the following to your `Dockerfile`:
+
+```Dockerfile
+COPY /etc/zope.conf.d/*.conf /app/etc/zope.conf.d/
+```
+
+This ensures your fragments are deployed in the `zope.conf.d` folder, which then
+will be used to amend the `zope.conf` file prior to starting Plone.
 
 ## Advanced usage