Merge pull request #2133 from nextcloud/enh/readme/persist-volumes-custom-upgrade-exclude

enh(README) Persistent volume upgrade.exclude clarification

Author: joshtrichards

README.md

@@ -71,6 +71,8 @@ $ docker run -d \
 mariadb:10.6
 ```
 
+### Additional volumes
+
 If you want to get fine grained access to your individual files, you can mount additional volumes for data, config, your theme and custom apps. The `data`, `config` files are stored in respective subfolders inside `/var/www/html/`. The apps are split into core `apps` (which are shipped with Nextcloud and you don't need to take care of) and a `custom_apps` folder. If you use a custom theme it would go into the `themes` subfolder.
 
 Overview of the folders that can be mounted as volumes:
@@ -91,10 +93,16 @@ $ docker run -d \
 -v theme:/var/www/html/themes/<YOUR_CUSTOM_THEME> \
 nextcloud
 ```
-If mounting additional volumes, you should note that data inside the main folder (`/var/www/html`) may be removed during installation and upgrades, unless listed in [upgrade.exclude](https://github.com/nextcloud/docker/blob/master/upgrade.exclude). You should consider:
+
+### Custom volumes
+
+If mounting additional volumes under `/var/www/html`, you should consider:
 - Confirming that [upgrade.exclude](https://github.com/nextcloud/docker/blob/master/upgrade.exclude) contains the files and folders that should persist during installation and upgrades; or 
 - Mounting storage volumes to locations outside of `/var/www/html`.
 
+> [!WARNING]
+> You should note that data inside the main folder (`/var/www/html`) will be overridden/removed during installation and upgrades, unless listed in [upgrade.exclude](https://github.com/nextcloud/docker/blob/master/upgrade.exclude). The additional volumes officially supported are already in that list, but custom volumes will need to be added by you. We suggest mounting custom storage volumes outside of `/var/www/html` and if possible read-only so that making this adjustment is unnecessary. If you must do so, however, you may build a custom image with a modified `/upgrade.exclude` file that incorporates your custom volume(s).
+
 
 ## Using the Nextcloud command-line interface
 To use the [Nextcloud command-line interface](https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/occ_command.html) (aka. `occ` command):