ci: use docs-builder image for deploy job

The docs builder uses root as the primary user of the container,
therefore should alleviate cache ownership issues.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

.gitlab-ci.yml

@@ -141,6 +141,8 @@ build-docs:
 
 deploy:
   extends: .oniro-repo-workspace
+  image:
+    name: registry.ostc-eu.org/ostc/oniro/docs-builder:latest
   stage: deploy
   script:
     # We are in the root of the git-repo workspace.