ci: initialize multibranch webhook fixture

Jenkinsfile

@@ -0,0 +1,33 @@
+pipeline {
+  agent any
+  options { timeout(time: 60, unit: 'SECONDS') }
+  stages {
+    stage('Checkout and identify change') {
+      steps {
+        checkout scm
+        echo "BRANCH_NAME=${env.BRANCH_NAME ?: ''}"
+        echo "CHANGE_ID=${env.CHANGE_ID ?: ''}"
+        echo "CHANGE_TARGET=${env.CHANGE_TARGET ?: ''}"
+        script {
+          def scenario = readFile('ci/scenario.txt').trim()
+          if (!(scenario in ['success', 'unstable', 'failure'])) {
+            error("Unsupported demo scenario: ${scenario}")
+          }
+          env.SCENARIO = scenario
+        }
+      }
+    }
+    stage('Build and report') {
+      steps {
+        script {
+          int status = sh(script: 'sh ci/demo-build.sh', returnStatus: true)
+          junit 'reports/*.xml'
+          if (status != 0) error("Demo build exited with status ${status}")
+        }
+      }
+    }
+  }
+  post {
+    always { archiveArtifacts artifacts: 'artifacts/*', allowEmptyArchive: true }
+  }
+}