The minimal Pipeline script results in an error:
java.nio.file.NoSuchFileException: /var/lib/jenkins/jobs/.../.../workspace

pipeline {
    agent any    stages {
        stage('Read file') {
            steps {
                script {
                    readFile("${WORKSPACE}")
                }
            }
        }
    }
}

Total unclear to me because readFile should be the method to read a file on the slave where new File("${WORKSPACE}") will allways read it on the master. Any hint?