ci: build docs using meson

To avoid having to inflate the image here even further, let's just add
what we need to the the normal x86 Alpine build image, and use that.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11494>

.gitlab-ci/container/gitlab-ci.yml

@@ -346,6 +346,12 @@ alpine/x86_64_build:
     - .alpine/x86_64_build-base
   variables:
     MESA_IMAGE_TAG: &alpine-x86_64_build ${ALPINE_X86_64_BUILD_TAG}
+  rules:
+    - !reference [pages, rules]
+    - !reference [test-docs, rules]
+    - !reference [test-docs-mr, rules]
+    - !reference [linkcheck-docs, rules]
+    - !reference [.container, rules]
 
 .use-alpine/x86_64_build:
   extends: