ci: add graphviz to the .docs-base template

The Gallium docs uses graphviz, so let's make sure it's installed first.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5706>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index be43e51..6f40995 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -36,7 +36,7 @@
   extends: .ci-run-policy
   image: alpine
   script:
-  - apk --no-cache add py3-pip
+  - apk --no-cache add py3-pip graphviz
   - pip3 install sphinx sphinx_rtd_theme
   - sphinx-build -b html docs public