Docker agent in Jenkinsfile - Step-by-Step Execution

Process Flow - Docker agent in Jenkinsfile

Start Jenkins Pipeline

↓

Define Docker Agent

↓

Jenkins pulls Docker image

↓

Run build steps inside Docker container

↓

Build steps execute commands

↓

Container stops after steps

↓

Pipeline ends

The Jenkins pipeline starts, defines a Docker agent, pulls the Docker image, runs build steps inside the container, then stops the container and ends.

Execution Sample

Jenkins

pipeline {
  agent {
    docker {
      image 'python:3.12'
    }
  }
  stages {
    stage('Test') {
      steps {
        sh 'python --version'
      }
    }
  }
}

This Jenkinsfile runs the 'Test' stage inside a Python 3.12 Docker container and prints the Python version.

Process Table

Step	Action	Evaluation	Result
1	Start pipeline	N/A	Pipeline begins execution
2	Define agent as Docker with image 'python:3.12'	N/A	Jenkins prepares to use Docker agent
3	Pull Docker image 'python:3.12'	Image not present locally	Image downloaded from Docker Hub
4	Start container from image	Container created	Container running
5	Execute step: sh 'python --version'	Run command inside container	Outputs 'Python 3.12.x'
6	Finish stage 'Test'	All steps done	Stage completed
7	Stop Docker container	Container stopped	Container removed
8	Pipeline ends	All stages complete	Pipeline finished successfully

💡 Pipeline ends after all stages complete and Docker container stops

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 7	Final
Docker Image	undefined	'python:3.12' pulled	Image ready	Container stopped	N/A
Container State	none	none	running	stopped	removed
Pipeline State	not started	started	running	running	finished

Key Moments - 3 Insights

Why does Jenkins pull the Docker image even if it might exist locally?

Jenkins checks if the image exists locally; if not, it pulls it. This is shown in step 3 where the image is downloaded only if missing.

What happens to the Docker container after the build steps finish?

As shown in step 7, the container stops and is removed to clean up resources after the pipeline stage completes.

How does Jenkins run commands inside the Docker container?

In step 5, Jenkins executes shell commands inside the running container, isolating the build environment.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Jenkins start the Docker container?

A Step 4

B Step 3

C Step 5

D Step 7

💡 Hint

Check the 'Action' column for 'Start container from image' in the execution table.

According to the variable tracker, what is the container state after step 7?

A removed

B running

C stopped

D none

💡 Hint

Look at the 'Container State' row under 'After Step 7' in the variable tracker.

If the Docker image is already present locally, which step in the execution table would change?

A Step 4 would be skipped

B Step 3 would be skipped or faster

C Step 5 would not run

D Step 7 would be earlier

💡 Hint

Step 3 is about pulling the image; if present locally, pulling is skipped or very fast.

Concept Snapshot

Jenkins Docker agent syntax:
agent { docker { image 'image_name' } }

Jenkins pulls the image if missing,
runs build steps inside the container,
then stops and removes the container.
Use for isolated, consistent build environments.

Full Transcript

This visual execution shows how Jenkins uses a Docker agent in a pipeline. The pipeline starts, defines a Docker agent with a specific image, and Jenkins pulls the image if it is not already present locally. Then Jenkins starts a container from that image and runs the build steps inside it. After the steps complete, the container stops and is removed. The pipeline then finishes successfully. Variables like Docker image, container state, and pipeline state change step-by-step. Key moments clarify why Jenkins pulls images, how commands run inside containers, and container cleanup. Quiz questions test understanding of container start, container state after stopping, and image pulling behavior.