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>

.github/CODEOWNERS

@@ -209,6 +209,7 @@
 /devenv/docker/blocks/tempo/ @grafana/observability-traces-and-profiling
 /devenv/docker/blocks/traefik/ @mckn
 /devenv/docker/blocks/zipkin/ @grafana/observability-traces-and-profiling
+/devenv/docker/blocks/webdav/ @grafana/alerting-backend-product
 /devenv/docker/buildcontainer/ @bergquist
 /devenv/docker/compose_header.yml @grafana/backend-platform
 /devenv/docker/debtest/ @bergquist