fix: branch_specifier is needed

.ci/build-docker-images.groovy

@@ -24,15 +24,23 @@ pipeline {
     rateLimitBuilds(throttle: [count: 60, durationName: 'hour', userBoost: true])
     quietPeriod(10)
   }
+  parameters {
+    string(name: 'branch_specifier', defaultValue: 'master', description: 'It would not be defined on the first build, see JENKINS-41929.')
+  }
   triggers {
-    cron 'H H(0-5) * * 1-5'
+    cron 'H H(4-5) * * 1-5'
   }
   stages {
     stage('Checkout') {
+      environment {
+        // Parameters will be empty for the very first build, setting an environment variable
+        // with the same name will workaround the issue. see JENKINS-41929
+        BRANCH_SPECIFIER = "${params?.branch_specifier}"
+      }
       steps {
         deleteDir()
         gitCheckout(basedir: "${BASE_DIR}",
-          branch: "${params.BRANCH_REFERENCE}",
+          branch: "${env.BRANCH_SPECIFIER}",
           repo: "https://github.com/elastic/${REPO}.git",
           credentialsId: "${JOB_GIT_CREDENTIALS}"
         )

.ci/jobs/build-docker-images.yml

@@ -18,5 +18,3 @@
             reference-repo: /var/lib/jenkins/.git-references/e2e-testing.git
             branches:
               - master
-    triggers:
-      - timed: 'H H(0-5) * * 1-5'