mix scripted with declarative

mma.groovy

pipeline {
    agent {
        docker { image 'docker-registry:5000/ruby:2.5.1' }
    }
    stages {
        stage('Test') {
            steps {
                script {
                    String target = 'bla'
                    String targetUser = 'ku'
                    sshagent (['test']) {
                        sh([
                            label: "on current agent",
                            script: "ls -l"
                        ])
                        sh([
                            label: "on ${target}",
                            script: "ssh -l ${targetUser} ${target} whoami"
                        ])
                    }
                }
            }
        }
    }
}