// Global option
pipeline {
  options {
    sshagent('credential-id')
  }
  stages {
    stage('ssh-deploy') {
      steps {
        sh 'ssh user@host command'
      }
    }
}

// option for single stage
pipeline {
  stages {
    stage('ssh-deploy') {
      options {
        sshagent('credential-id')
      }
      steps {
        sh 'ssh user@host command'
      }
    }
  }
}

// option for single stage + docker
pipeline {
 stages {
   stage('ssh-deploy') {
     agent {
       docker { image 'ubuntu:16.04' }
     }
     options {
       sshagent('credential-id')
     }
     steps {
       sh 'ssh user@host command'
     }
    }
  }
}