Echo build number

Jenkinsfile

@@ -12,6 +12,7 @@ pipeline {
             steps {
                 script {
                     def buildNumber = env.BUILD_NUMBER
+                    sh 'echo ${buildNumber}'
                     def dockerImage = docker.build('git.thisfro.ch/jenkinsci/pflaenzli', '-f Dockerfile . --tag git.thisfro.ch/jenkinsci/pflaenzli:${buildNumber}')
                 }
             }