Building Docker images in pipeline in Jenkins - Deep Dive

Overview - Building Docker images in pipeline

What is it?

Building Docker images in a pipeline means automating the process of creating Docker containers using a series of steps defined in a Jenkins pipeline. This allows developers to package their applications and dependencies into a single image that can run anywhere. The pipeline runs commands to build the image, test it, and prepare it for deployment without manual intervention. It makes software delivery faster and more reliable.

Why it matters

Without building Docker images in a pipeline, developers would have to manually create and manage container images, which is slow and error-prone. Automating this process ensures consistency, repeatability, and speed, reducing human mistakes and enabling continuous delivery. This means new features and fixes reach users faster and with higher quality.

Where it fits

Before learning this, you should understand basic Docker concepts like images, containers, and Dockerfiles, as well as Jenkins pipeline basics. After mastering this, you can learn about deploying Docker images to cloud platforms, managing multi-stage builds, and integrating security scans in pipelines.

Mental Model

Core Idea

A Jenkins pipeline automates the step-by-step process of creating, testing, and preparing Docker images so software can be reliably packaged and delivered.

Think of it like...

Building Docker images in a pipeline is like an automated sandwich-making machine in a restaurant kitchen: it follows a recipe step-by-step without mistakes, producing the same sandwich every time quickly and reliably.

Pipeline Start
   │
   ▼
[Checkout Code]
   │
   ▼
[Build Docker Image]
   │
   ▼
[Test Image]
   │
   ▼
[Push Image to Registry]
   │
   ▼
Pipeline End

Build-Up - 7 Steps

1

Foundation

Understanding Docker Image Basics

Concept: Learn what a Docker image is and how it is created using a Dockerfile.

A Docker image is a snapshot of an application and its environment. It is built using instructions in a Dockerfile, which tells Docker how to assemble the image step-by-step. For example, a Dockerfile might start from a base image, copy application files, install dependencies, and set commands to run the app.

Result

You understand that a Docker image is a reusable package that contains everything needed to run an application.

Knowing what a Docker image is helps you see why automating its creation is important for consistent software delivery.

2

Foundation

Basics of Jenkins Pipeline Syntax

Concept: Learn how Jenkins pipelines are written to automate tasks using stages and steps.

A Jenkins pipeline is a script that defines stages like 'Build', 'Test', and 'Deploy'. Each stage contains steps that run commands or scripts. Pipelines can be written in a simple syntax called Declarative Pipeline or a more flexible one called Scripted Pipeline.

Result

You can write a basic Jenkins pipeline that runs commands in order.

Understanding pipeline structure is key to automating Docker image builds reliably.

3

Intermediate

Building Docker Images in Jenkins Pipeline

🤔Before reading on: do you think Jenkins needs Docker installed on the same machine to build images? Commit to your answer.

Concept: Learn how to add Docker build commands inside Jenkins pipeline stages to create images automatically.

In a pipeline stage, you use shell commands like 'docker build -t myimage:tag .' to build the image from the Dockerfile. Jenkins must run on a machine with Docker installed or use a Docker agent. You tag the image to identify it later.

Result

The pipeline builds a Docker image automatically every time it runs.

Knowing how to run Docker commands inside Jenkins connects containerization with automation, speeding up development.

4

Intermediate

Pushing Docker Images to a Registry

🤔Before reading on: do you think pushing images requires authentication? Commit to your answer.

Concept: Learn how to push the built Docker image to a remote registry like Docker Hub or a private registry.

After building, use 'docker push myimage:tag' to upload the image to a registry. You must log in first using 'docker login' with credentials stored securely in Jenkins. This makes the image available for deployment elsewhere.

Result

The image is stored remotely and can be pulled by other systems.

Understanding image pushing enables sharing and deploying containers across environments.

5

Intermediate

Using Jenkins Docker Pipeline Plugin

Concept: Learn how Jenkins plugins simplify Docker commands and improve pipeline readability.

The Jenkins Docker Pipeline plugin provides special steps like 'docker.build()' and 'docker.image()' to build and manage images without raw shell commands. For example, 'def img = docker.build("myimage:tag")' builds and stores the image object for later use.

Result

Pipeline scripts become cleaner and easier to maintain.

Using plugins abstracts complexity and reduces errors in pipeline scripts.

6

Advanced

Multi-Stage Docker Builds in Pipelines

🤔Before reading on: do you think multi-stage builds reduce image size or increase it? Commit to your answer.

Concept: Learn how to use multi-stage Dockerfiles in pipelines to create smaller, efficient images.

Multi-stage builds use multiple FROM statements in a Dockerfile to separate build and runtime environments. The pipeline builds the image as usual, but the final image only contains necessary runtime files, making it smaller and faster to deploy.

Result

The pipeline produces optimized Docker images with less bloat.

Knowing multi-stage builds improves performance and security of containerized apps.

7

Expert

Handling Docker Build Secrets and Caching

🤔Before reading on: do you think Docker build cache always speeds up builds? Commit to your answer.

Concept: Learn advanced techniques to securely pass secrets during builds and optimize build caching in pipelines.

Docker build secrets let you pass sensitive data like API keys without storing them in images. You can use '--secret' flags in build commands. Build cache reuses unchanged layers to speed up builds, but cache invalidation can cause unexpected rebuilds. Pipelines can be configured to manage cache and secrets carefully.

Result

Builds are faster and more secure in production pipelines.

Understanding secrets and caching prevents security leaks and improves pipeline efficiency.

Under the Hood

When a Jenkins pipeline runs a Docker build, it sends instructions from the Dockerfile to the Docker daemon on the build machine. The daemon executes each step, creating image layers stored locally. The pipeline controls this process by running shell commands or plugin steps. When pushing, the daemon uploads layers to the registry. Jenkins orchestrates these steps, managing credentials and environment.

Why designed this way?

Separating the pipeline orchestration from Docker's build engine allows flexibility and security. Jenkins focuses on automation and workflow, while Docker handles image creation. This separation lets teams use different build machines and keeps credentials isolated. Plugins were created to simplify common Docker tasks and reduce errors in scripts.

Jenkins Pipeline
   │
   ▼
[Pipeline Script]
   │
   ▼
[Docker CLI Commands]
   │
   ▼
Docker Daemon (Build Engine)
   │
   ▼
[Image Layers Stored Locally]
   │
   ▼
[Push to Registry]
   │
   ▼
[Remote Registry Storage]

Myth Busters - 4 Common Misconceptions

Quick: Does Jenkins automatically have Docker installed on all agents? Commit yes or no.

Common Belief: Jenkins agents always have Docker installed and ready to use.

Reality: Jenkins agents need Docker installed and properly configured to run Docker commands; otherwise, builds fail.

Why it matters: Assuming Docker is present causes pipeline failures and wasted debugging time.

Quick: Can you push Docker images without logging into the registry? Commit yes or no.

Common Belief: You can push Docker images to any registry without authentication.

Reality: Most registries require authentication before pushing images to prevent unauthorized uploads.

Why it matters: Skipping login causes push failures and blocks deployment.

Quick: Does using multi-stage builds always make images bigger? Commit yes or no.

Common Belief: Multi-stage Docker builds increase image size because they add more steps.

Reality: Multi-stage builds reduce image size by discarding build-only layers and keeping only runtime essentials.

Why it matters: Misunderstanding this leads to missing out on performance and security benefits.

Quick: Does Docker build cache always speed up builds? Commit yes or no.

Common Belief: Docker build cache always makes builds faster without downsides.

Reality: Cache can speed up builds but may cause stale layers if dependencies change unnoticed, leading to bugs.

Why it matters: Ignoring cache invalidation can cause hard-to-find errors in production images.

Expert Zone

1

Jenkins pipelines can use Docker-in-Docker or Docker socket binding; each has security and stability tradeoffs that experts carefully evaluate.

2

Managing credentials securely in Jenkins using credentials plugins and environment variables prevents leaks during Docker login steps.

3

Optimizing layer order in Dockerfiles affects build cache efficiency and image size, a subtlety often overlooked.

When NOT to use

Building Docker images in Jenkins pipelines is not ideal when you need ultra-fast iterative builds; in such cases, local builds or specialized build tools like BuildKit or Kaniko may be better. Also, for very simple apps, manual builds might suffice.

Production Patterns

In production, teams use multi-branch pipelines that build images per feature branch, tag images with commit hashes, scan images for vulnerabilities automatically, and deploy only images passing tests. They also use shared build agents with Docker caching and clean up old images regularly.

Connections

Continuous Integration

Building Docker images in pipelines is a key step in continuous integration workflows.

Understanding Docker image builds helps grasp how automated testing and packaging fit into the larger CI process.

Immutable Infrastructure

Docker images built in pipelines embody immutable infrastructure principles by packaging apps as unchangeable units.

Knowing this connection clarifies why container images improve deployment reliability and rollback safety.

Manufacturing Assembly Lines

The pipeline's step-by-step Docker image build process mirrors assembly lines in manufacturing.

Seeing pipelines as assembly lines highlights the importance of automation, quality checks, and repeatability in software delivery.

Common Pitfalls

#1 Trying to build Docker images on Jenkins agents without Docker installed.

Wrong approach:

pipeline {
agent any
stages {
stage('Build') {
steps {
sh 'docker build -t myapp:latest .'
}
}
}
}

Correct approach:

pipeline {
agent { docker { image 'docker:20.10' args '-v /var/run/docker.sock:/var/run/docker.sock' } }
stages {
stage('Build') {
steps {
sh 'docker build -t myapp:latest .'
}
}
}
}

Root cause:

Assuming the default Jenkins agent has Docker installed leads to build failures.

#2 Pushing Docker images without logging into the registry first.

Wrong approach:

sh 'docker push myrepo/myapp:latest'

Correct approach:

withCredentials([usernamePassword(credentialsId: 'docker-creds', usernameVariable: 'USER', passwordVariable: 'PASS')]) {
sh 'docker login -u $USER -p $PASS'
sh 'docker push myrepo/myapp:latest'
}

Root cause:

Forgetting authentication steps causes push commands to fail.

#3 Writing Dockerfile stages in wrong order causing cache misses.

Wrong approach:

FROM node:14
COPY . /app
RUN npm install
CMD ["node", "app.js"]

Correct approach:

FROM node:14
COPY package.json /app/
RUN npm install
COPY . /app
CMD ["node", "app.js"]

Root cause:

Not ordering Dockerfile instructions to maximize cache reuse slows builds.

Key Takeaways

Building Docker images in Jenkins pipelines automates packaging applications into portable containers, improving speed and reliability.

Jenkins pipelines run Docker commands or use plugins to build, test, and push images as part of continuous integration.

Proper setup of Docker on Jenkins agents and secure handling of credentials are essential for successful builds.

Advanced techniques like multi-stage builds and build secrets optimize image size and security.

Understanding pipeline automation and Docker internals helps prevent common mistakes and improves production workflows.