gitlab-ci: Add needs: for x86 buster docker image

This allows most build jobs to run before the stretch or arm64 docker
images are ready.

v2:
* Use - list syntax instead of [] (Eric Engestrom)

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

.gitlab-ci.yml

@@ -94,6 +94,8 @@ test-container:arm64:
 .build:
   extends: .ci-run-policy
   image: $DEBIAN_IMAGE
+  needs:
+    - debian-10
   stage: build
   cache:
     paths: