CI: docs: fail on warning (MR 2506)

.ci/docs.sh

@@ -32,6 +32,7 @@ if [ "$fail" -eq 1 ]; then
 fi
 
 sphinx-build \
+	--fail-on-warning \
 	docs \
 	public \
 

.gitlab-ci.yml

@@ -90,6 +90,9 @@ mypy:
 
 docs:
   stage: test
+  # py3-sphinx >= 7.3 is needed for --fail-on-warning
+  # alpine:3.20 is on py3-sphinx 7.2.x
+  image: alpine:edge
   script:
     - ".ci/docs.sh"
   artifacts: