docs(datasource/docker): add `RENOVATE_X_DOCKER_HUB_TAGS` (#26185)

Co-authored-by: Rhys Arkins <rhys@arkins.net>

docs/usage/self-hosted-experimental.md

@@ -80,6 +80,10 @@ You can set the config file Renovate should read with the `RENOVATE_CONFIG_FILE`
 
 The process that runs Renovate must have the correct permissions to delete the config file.
 
+## `RENOVATE_X_DOCKER_HUB_TAGS`
+
+If set to any value, Renovate will use the Docker Hub API (`https://hub.docker.com`) to fetch tags instead of the normal Docker API for images pulled from `https://index.docker.io`.
+
 ## `RENOVATE_X_DOCKER_MAX_PAGES`
 
 If set to an integer, Renovate will use this as max page number for docker tags lookup on docker registries, instead of the default 20 pages.