chore: add parameter in the pipeline

.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(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

@@ -5,11 +5,6 @@
     description: Job to pre-build docker images used in E2E tests.
     view: Beats
     project-type: pipeline
-    parameters:
-      - string:
-          name: branch_specifier
-          default: master
-          description: the Git branch specifier to build
     pipeline-scm:
       script-path: .ci/build-docker-images.groovy
       scm:
@@ -22,4 +17,4 @@
             credentials-id: f6c7695a-671e-4f4f-a331-acdce44ff9ba
             reference-repo: /var/lib/jenkins/.git-references/e2e-testing.git
             branches:
-              - $branch_specifier
+              - master