ci: trigger creation of disk images

This ci job will trigger a downstream multi-project pipeline to
build a disk image.

.gitlab-ci.yml

 stages:
   - verify
+  - trigger
 
 variables:
   GIT_STRATEGY: clone
@@ -115,3 +116,13 @@ build-riscv64-emulated:
     - docker-alpine
     - ci-build
     - riscv64
+
+trigger_build_image:
+  stage: trigger
+  only:
+    - tags
+  variables:
+    APORTS_TAG: $CI_COMMIT_TAG
+  trigger:
+    project: clandmeter/alpine-disk-image
+    branch: master