docker: Alpine build job depends on Alpine container

Add missing dependency build-system-alpine -> amd64-alpine-container.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210303130646.1494015-2-philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.gitlab-ci.yml

@@ -76,6 +76,8 @@ include:
 
 build-system-alpine:
   <<: *native_build_job_definition
+  needs:
+    - job: amd64-alpine-container
   variables:
     IMAGE: alpine
     TARGETS: aarch64-softmmu alpha-softmmu cris-softmmu hppa-softmmu