Move Auto DevOps Build.gitlab-ci.yml to rules

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/213336

changelogs/unreleased/move-build-template-to-rules-syntax.yml

+---
+title: Move Build.gitlab-ci.yml to `rules` syntax
+merge_request: 30895
+author:
+type: changed

lib/gitlab/ci/templates/Jobs/Build.gitlab-ci.yml

@@ -15,6 +15,5 @@ build:
         export CI_APPLICATION_TAG=${CI_APPLICATION_TAG:-$CI_COMMIT_TAG}
       fi
     - /build/build.sh
-  only:
-    - branches
-    - tags
+  rules:
+    - if: '$CI_COMMIT_TAG || $CI_COMMIT_BRANCH'

spec/lib/gitlab/ci/templates/Jobs/build_gitlab_ci_yaml_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Jobs/Build.gitlab-ci.yml' do
+  subject(:template) { Gitlab::Template::GitlabCiYmlTemplate.find('Jobs/Build') }
+
+  describe 'the created pipeline' do
+    let_it_be(:user) { create(:admin) }
+    let_it_be(:project) { create(:project, :repository) }
+
+    let(:default_branch) { 'master' }
+    let(:pipeline_ref) { default_branch }
+    let(:service) { Ci::CreatePipelineService.new(project, user, ref: pipeline_ref) }
+    let(:pipeline) { service.execute!(:push) }
+    let(:build_names) { pipeline.builds.pluck(:name) }
+
+    before do
+      stub_ci_pipeline_yaml_file(template.content)
+      allow_any_instance_of(Ci::BuildScheduleWorker).to receive(:perform).and_return(true)
+      allow(project).to receive(:default_branch).and_return(default_branch)
+    end
+
+    context 'on master' do
+      it 'creates the build job' do
+        expect(build_names).to contain_exactly('build')
+      end
+    end
+
+    context 'on another branch' do
+      let(:pipeline_ref) { 'feature' }
+
+      it 'creates the build job' do
+        expect(build_names).to contain_exactly('build')
+      end
+    end
+
+    context 'on tag' do
+      let(:pipeline_ref) { 'v1.0.0' }
+
+      it 'creates the build job' do
+        expect(pipeline).to be_tag
+        expect(build_names).to contain_exactly('build')
+      end
+    end
+
+    context 'on merge request' do
+      let(:service) { MergeRequests::CreatePipelineService.new(project, user) }
+      let(:merge_request) { create(:merge_request, :simple, source_project: project) }
+      let(:pipeline) { service.execute(merge_request) }
+
+      it 'has no jobs' do
+        expect(pipeline).to be_merge_request_event
+        expect(build_names).to be_empty
+      end
+    end
+  end
+end