Docker agents for isolation in Jenkins - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Jenkins pipeline with Docker agent

What will be the output of this Jenkins pipeline snippet when run on a Jenkins master with Docker installed?

Jenkins

pipeline {
  agent {
    docker {
      image 'alpine:3.14'
      args '-v /tmp:/tmp'
    }
  }
  stages {
    stage('Test') {
      steps {
        sh 'echo Hello from inside Docker'
      }
    }
  }
}

A Hello from inside Docker

B Error: Docker daemon not found

C Permission denied when accessing /tmp

D Pipeline skipped due to missing image

💡 Hint

The pipeline uses a Docker agent with Alpine image and runs a simple echo command inside the container.

The pipeline runs the shell command inside the Alpine Docker container, so the output is the echo statement's text.

❓ Configuration

intermediate

Correct Docker agent syntax in Jenkinsfile

Which Jenkinsfile snippet correctly defines a Docker agent that uses the 'node:18' image and mounts the host directory '/data' to '/app/data' inside the container?

A

agent {
  docker {
    image 'node:18'
    args '-v /data:/app/data'
  }
}

B

agent {
  docker {
    image 'node:18'
    args '--mount /data:/app/data'
  }
}

C

agent {
  docker {
    image 'node:18'
    args '--volume /data:/app/data'
  }
}

D

agent {
  docker {
    image 'node:18'
    args '-m /data:/app/data'
  }
}

💡 Hint

Docker volume mount syntax uses '-v' flag.

The correct Docker run argument to mount a volume is '-v host_path:container_path'.

❓ Troubleshoot

advanced

Troubleshooting Docker agent permission error

A Jenkins pipeline using a Docker agent fails with the error: 'permission denied while trying to connect to the Docker daemon socket'. What is the most likely cause?

A The Docker image specified does not exist in the registry.

B The Jenkins user does not have permission to access the Docker socket on the host.

C The Jenkinsfile syntax for the Docker agent is incorrect.

D The Docker daemon is running but out of disk space.

💡 Hint

Accessing Docker requires permission to the Docker socket file.

The error indicates Jenkins cannot access the Docker daemon socket, usually due to user permission issues.

🔀 Workflow

advanced

Jenkins pipeline with multiple Docker agents

In a Jenkins pipeline, you want to run one stage inside a Python Docker container and another stage inside a Node.js Docker container. Which pipeline structure correctly achieves this?

A

pipeline {
  agent { docker { image 'node:18' } }
  stages {
    stage('Python Stage') {
      agent { docker { image 'python:3.10' } }
      steps { sh 'python --version' }
    }
    stage('Node Stage') {
      steps { sh 'node --version' }
    }
  }
}

B

pipeline {
  agent { docker { image 'python:3.10' } }
  stages {
    stage('Python Stage') {
      steps { sh 'python --version' }
    }
    stage('Node Stage') {
      agent { docker { image 'node:18' } }
      steps { sh 'node --version' }
    }
  }
}

C

pipeline {
  agent none
  stages {
    stage('Python Stage') {
      agent { docker { image 'python:3.10' } }
      steps { sh 'python --version' }
    }
    stage('Node Stage') {
      agent { docker { image 'node:18' } }
      steps { sh 'node --version' }
    }
  }
}

D

pipeline {
  agent none
  stages {
    stage('Python Stage') {
      steps { sh 'docker run python:3.10 python --version' }
    }
    stage('Node Stage') {
      steps { sh 'docker run node:18 node --version' }
    }
  }
}

💡 Hint

Use 'agent none' at top level to specify agents per stage.

Defining 'agent none' at pipeline level allows each stage to specify its own Docker agent.

✅ Best Practice

expert

Best practice for Docker agent cleanup in Jenkins

Which approach ensures Docker containers used as Jenkins agents are properly cleaned up after pipeline execution to avoid resource leaks?

A Manually run 'docker rm -f' commands in pipeline steps after each stage.

B Configure Jenkins to use 'docker' agents with 'alwaysPull true' to refresh images each run.

C Use 'agent any' and run Docker containers outside Jenkins to avoid cleanup issues.

D Use the 'docker' agent with 'reuseNode false' and rely on Jenkins to remove containers automatically.

💡 Hint

Jenkins Docker plugin can manage container lifecycle automatically.

Setting 'reuseNode false' tells Jenkins to remove the container after the build, preventing leftover containers.