gitlab-ci.yml: move build-docs to build stage

The implicit test stage is a scheduled to run after all the jobs from
the build stage succeed. This introduces a unneeded dependency between
building the code and building the documentation.

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

.ostc-ci/gitlab-ci.yml

@@ -44,6 +44,7 @@ reuse:
 build-docs:
   image:
     name: registry.ostc-eu.org/ostc/containers/ostc-docs-builder
+  stage: build
   script:
     - cd docs && make
   artifacts: