imagestorage: fix templating of public_url option for webdav (#74885)

* spelling in error message

* imguploader: fix template for webdav url

Since a few years ago, when loading the config file, the  template gets replaced in the expanding
env var phase (see ##25075) and introduced a bug in the webdav public_url config option. This commit
changes the template syntax to be {{file}}. This doesn't get expanded and removed when the config file
is loaded.

* devenv: add a block for a WebDAV server

* Adding alerting as codeowners for webdav

---------

Co-authored-by: Timur Olzhabayev <timur.olzhabayev@grafana.com>

devenv/docker/blocks/webdav/README.md

@@ -0,0 +1,26 @@
+# README for the image storage WebDAV docker block
+
+This block is used for testing the [WebDAV](https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/#external_image_storagewebdav) option for external image storage which is used in [alert notifications](https://grafana.com/docs/grafana/latest/alerting/manage-notifications/images-in-notifications/). This uses the simplest WebDav server that is still being maintained, a project called [Dufs](https://github.com/sigoden/dufs).
+
+## Using Dufs
+
+Dufs has a web UI that can be accessed at http://localhost:5000 to easily see which files have been uploaded by Grafana. The config has disabled authentication and allows everyone to read and write files.
+
+## Configuring image storage in Grafana to use Dufs
+
+An example config for external image storage with webdav enabled:
+
+```ini
+[external_image_storage]
+provider = webdav
+
+[external_image_storage.webdav]
+url = http://127.0.0.1:5000/images
+public_url = http://127.0.0.1:5000/images/{{file}}
+
+; as auth is not configured in Dufs, these are just dummy values
+username = test
+password = test
+```
+
+Note: As everything runs on localhost, the image in an email notification will be broken but the link will work if you click on it and open it in your browser.

devenv/docker/blocks/webdav/docker-compose.yaml

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+  webdav:
+    image: sigoden/dufs
+    ports:
+      - "5000:5000"
+    volumes:
+      - "webdav-data:/data"
+    command: "/data -A"
+
+volumes:
+  webdav-data: {}