ci: use dynamic downstream pipelines

This gives us more flexibiity with regards to the job configuration. One
job generates the job configuration which then triggers a downstream
pipeline.

.gitlab-ci.yml

@@ -7,10 +7,10 @@ variables:
   GIT_DEPTH: "500"
   REGISTRY: registry.alpinelinux.org
 
-# only rune pipelines for merge requests or when tags are pushed  (for triggers)
 workflow:
   rules:
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_PIPELINE_SOURCE == "pipeline" || $CI_PIPELINE_SOURCE == "parent_pipeline"
     - if: $CI_COMMIT_TAG
 
 lint:
@@ -26,6 +26,32 @@ lint:
     - docker-alpine
     - x86_64
 
+generate-build-jobs:
+  stage: verify
+  image:
+    name: $REGISTRY/alpine/infra/docker/alpine-gitlab-ci:latest
+  script:
+    - pipeline generate .gitlab-ci.yml .gitlab/job-config.yaml >.gitlab/build-jobs.yaml
+  artifacts:
+    paths:
+      - .gitlab/build-jobs.yaml
+  tags:
+    - docker-alpine
+    - x86_64
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+
+build-jobs:
+  stage: verify
+  needs:
+    - generate-build-jobs
+  trigger:
+    include:
+      - artifact: .gitlab/build-jobs.yaml
+        job: generate-build-jobs
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+
 .build:
   stage: verify
   image:
@@ -41,7 +67,7 @@ lint:
     expire_in: 1 day
     when: always
   rules:
-    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_PIPELINE_SOURCE == "parent_pipeline"
 
 build-x86_64:
   extends: .build