Docker agent in Jenkinsfile - Mini Project: Build & Apply

Using Docker Agent in Jenkinsfile

📖 Scenario: You are setting up a Jenkins pipeline to build and test your application inside a Docker container. This helps keep your build environment clean and consistent, just like using a special kitchen for baking cakes so the recipe always turns out the same.

🎯 Goal: Create a Jenkinsfile that uses a Docker agent with the agent block. You will specify the Docker image, add a simple environment variable, run a shell command inside the container, and print the output.

📋 What You'll Learn

Create a pipeline block with a agent using Docker image python:3.12-slim

Add an environment variable MY_VAR with value hello

Inside the stages, create a stage named Run Python that runs python --version and echo $MY_VAR

Print the output of the commands in the console

💡 Why This Matters

🌍 Real World

Using Docker agents in Jenkins pipelines helps developers run builds and tests in clean, consistent environments without installing tools on the Jenkins server itself.

💼 Career

Many DevOps and CI/CD engineer roles require writing Jenkins pipelines that use Docker containers to ensure reliable and repeatable builds.

1

Create the Jenkins pipeline with Docker agent

Write a Jenkinsfile starting with pipeline block that uses agent with Docker image python:3.12-slim.

Jenkins

pipeline {
    agent {
        // Use Docker image here
    }
    // Your code here
}

Need a hint?

Use agent { docker { image 'python:3.12-slim' } } inside the pipeline block.

2

Add environment variable inside the pipeline

Add an environment block inside the pipeline with variable MY_VAR set to hello.

Jenkins

pipeline {
    agent {
        docker {
            image 'python:3.12-slim'
        }
    }
    // Add environment variable here
}

Need a hint?

Use environment { MY_VAR = 'hello' } inside the pipeline block.

3

Add a stage to run commands inside Docker container

Add a stages block with a stage named Run Python. Inside it, add a steps block that runs shell commands python --version and echo $MY_VAR using sh.

Jenkins

pipeline {
    agent {
        docker {
            image 'python:3.12-slim'
        }
    }
    environment {
        MY_VAR = 'hello'
    }
    // Add stages here
}

Need a hint?

Use stages { stage('Run Python') { steps { sh 'python --version'; sh 'echo $MY_VAR' } } }.

4

Print the output of the commands

Run the Jenkins pipeline. The console output should show the Python version and the text hello printed from the environment variable.

Jenkins

pipeline {
    agent {
        docker {
            image 'python:3.12-slim'
        }
    }
    environment {
        MY_VAR = 'hello'
    }
    stages {
        stage('Run Python') {
            steps {
                sh 'python --version'
                sh 'echo $MY_VAR'
            }
        }
    }
}

// Run the pipeline and check output

Need a hint?

Look at the Jenkins console output after running the pipeline. It should show the Python version and the word hello.