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Michelle
Michelle

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30-Day Cloud & DevOps Challenge: Day 10 — Docker in CI

#automation #cicd #devops #docker

Yesterday, my Jenkins pipeline could install dependencies and build the frontend.

But there was a missing piece: Docker. Without it, I couldn't package my applications into containers — the whole point of this challenge!

Today, I fixed that. I configured Jenkins to build Docker images for both my backend and frontend, turning my CI pipeline into a complete build system.

First: Why Docker in CI?

Before Docker in CI

The pipeline could:

  • Pull code from GitHub
  • Install dependencies
  • Build the frontend
  • Could NOT create Docker images

After Docker in CI

The pipeline can:

  • Pull code from GitHub
  • Install dependencies
  • Build the frontend
  • Create Docker images for backend AND frontend

Why this matters: Docker images are what actually get deployed to production. Without them, you can't run your app anywhere else.

Step 1: Giving Jenkins Docker Access

The Problem

Jenkins runs in a container. By default, containers can't access the host's Docker daemon.

+------------------+     +------------------+
|     Jenkins      |     |   Docker Daemon  |
|    Container     |  X  |    (on host)     |
| "I need Docker"  |---->|  "Cannot reach"  |
+------------------+     +------------------+
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The Solution: Docker Socket Mounting

Mount the host's Docker socket into the Jenkins container:

# jenkins/docker-compose.yml
services:
  jenkins:
    user: root                      # Run as root for permissions
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock  # Mount Docker socket
    environment:
      - DOCKER_HOST=unix:///var/run/docker.sock   # Tell Jenkins where Docker is
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After Mounting 

+------------------+     +------------------+
|     Jenkins      |     |   Docker Daemon  |
|    Container     |  ✓  |    (on host)     |
| "I need Docker"  |---->|  "Here you go!"  |
+------------------+     +------------------+
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Step 2: Installing Docker in Jenkins Container

Even with the socket mounted, Jenkins needs the Docker CLI tool.

# Enter Jenkins container
sudo docker exec -it jenkins bash

# Install Docker CLI
apt-get update
apt-get install -y docker.io

# Verify it works
docker --version
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Expected output:

Docker version 26.1.5, build a72d7cd
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Step 3: Adding Docker Build to Pipeline

The Docker Build Stage 

stage('Docker Build') {
    steps {
        echo 'Building Docker images...'
        sh 'docker build -t myapp-backend:latest ./backend'
        sh 'docker build -t myapp-frontend:latest ./frontend'
    }
}
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What Each Command Does

Command What it does
docker build Creates a Docker image from a Dockerfile
-t myapp-backend:latest Tags the image with a name and version
./backend Tells Docker where to find the Dockerfile

The Complete Jenkinsfile (Docker Stage Added) 

pipeline {
    agent any

    stages {
        stage('Checkout') {
            steps {
                echo 'Cloning repository...'
                checkout scm
            }
        }

        stage('Backend Dependencies') {
            steps {
                dir('backend') {
                    sh 'npm install'
                }
            }
        }

        stage('Frontend Build') {
            steps {
                dir('frontend') {
                    sh 'npm install'
                    sh 'npm run build'
                }
            }
        }

        stage('Docker Build') {
            steps {
                echo 'Building Docker images...'
                sh 'docker build -t myapp-backend:latest ./backend'
                sh 'docker build -t myapp-frontend:latest ./frontend'
            }
        }

        stage('Success') {
            steps {
                echo 'Pipeline completed successfully!'
            }
        }
    }
}
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Step 4: Understanding Docker Build in CI

What Happens During Docker Build 

Stage: Docker Build
    │
    ├── Backend Image
    │   ├── FROM node:18-alpine
    │   ├── WORKDIR /app
    │   ├── COPY package*.json ./
    │   ├── RUN npm install
    │   ├── COPY . .
    │   └── CMD ["npm", "start"]
    │
    └── Frontend Image
        ├── FROM nginx:alpine
        ├── COPY build /usr/share/nginx/html
        └── EXPOSE 80
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Image Sizes

Image Size Why
myapp-backend:latest ~135MB Node.js + dependencies
myapp-frontend:latest ~23MB Just nginx + static files

Step 5: Testing Docker Build Locally

Before trusting Jenkins, test manually:

# Build backend image
cd backend
docker build -t test-backend .
docker run -p 3001:5000 test-backend
curl http://localhost:3001/health

# Build frontend image
cd ../frontend
npm run build
docker build -t test-frontend .
docker run -p 8080:80 test-frontend
# Open browser to http://localhost:8080
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Step 6: Running the Pipeline

Trigger the Build

  1. Go to Jenkins -> microservices-ci
  2. Click "Build Now"
  3. Watch the console output

Console Output 

[Pipeline] stage
[Pipeline] { (Docker Build)
[Pipeline] echo
Building Docker images...
[Pipeline] sh
+ docker build -t myapp-backend:latest ./backend
#1 [internal] load build definition from dockerfile
#1 transferring dockerfile: 149B done
#1 DONE 0.2s
#2 [internal] load metadata for docker.io/library/node:18-alpine
#2 DONE 35.8s
#3 [1/5] FROM docker.io/library/node:18-alpine
#3 DONE 0.0s
...
Successfully built dd22467f3082
Successfully tagged myapp-backend:latest

[Pipeline] sh
+ docker build -t myapp-frontend:latest ./frontend
#1 [internal] load build definition from dockerfile
...
Successfully built 54515d7d59a9
Successfully tagged myapp-frontend:latest

[Pipeline] }
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Success)
[Pipeline] echo
Pipeline completed successfully!
Finished: SUCCESS
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Docker in CI: Before vs After

Aspect Before (Day 9) After (Day 10)
Docker access No Yes
Backend image Not built Built automatically
Frontend image Not built Built automatically
Ready for deployment No Yes

Troubleshooting

Issue 1: "docker: command not found"

Error:

+ docker build
docker: command not found
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Fix: Install Docker CLI in Jenkins container:

sudo docker exec jenkins bash -c "apt-get update && apt-get install -y docker.io"
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Issue 2: "Permission denied"

Error:

Got permission denied while trying to connect to the Docker daemon
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Fix: Run Jenkins as root user in docker-compose.yml:

services:
  jenkins:
    user: root
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Issue 3: Build context issues

Error:

COPY failed: file not found in build context
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Fix: Check your Dockerfile paths. The build context is the directory you specify in docker build -t name ./path.

Key Takeaways

  • Docker in CI = Deployable artifacts — Without Docker, you can't deploy
  • Mount the Docker socket — Jenkins container needs access to host's Docker
  • Install Docker CLI — The container needs the docker command
  • Tag images meaningfully — latest is fine, but version tags are better
  • Build once, deploy anywhere — CI builds images, they can run anywhere

Resources

Let's Connect!

Have you integrated Docker with Jenkins before? What challenges did you face with Docker in CI?

Drop a comment or connect on LinkedIn. Let's learn together!

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