docs(gitlab): add hostType to registry hostRules guide (#22055)

lib/modules/platform/gitlab/index.md

@@ -29,7 +29,7 @@ Remember to set `platform=gitlab` somewhere in your Renovate config file.
 
 If you're using a private [GitLab container registry](https://docs.gitlab.com/ee/user/packages/container_registry/), you must:
 
-- Set the `RENOVATE_HOST_RULES` CI variable to `[{"matchHost": "${CI_REGISTRY}","username": "${GITLAB_USER_NAME}","password": "${RENOVATE_TOKEN}"}]`.
+- Set the `RENOVATE_HOST_RULES` CI variable to `[{"matchHost": "${CI_REGISTRY}","username": "${GITLAB_USER_NAME}","password": "${RENOVATE_TOKEN}", "hostType": "docker"}]`.
 - Make sure the user that owns the `RENOVATE_TOKEN` PAT is a member of the corresponding GitLab projects/groups with the right permissions.
 - Make sure the `RENOVATE_TOKEN` PAT has the `read_registry` scope.