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In this guide, we’ll walk through building a Docker image in a Jenkins CI/CD pipeline, tagging each build with the Git commit SHA for traceability.

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Add the Build Docker Image Stage

Open your Jenkinsfile and, right after the SonarQube stage, insert a new stage named Build Docker Image. This stage prints all environment variables and then builds the Docker image using the current Git commit hash (GIT_COMMIT) as the tag. 
stage('Build Docker Image') {
    steps {
        // Print all available environment variables
        sh 'printenv'
        
        // Build and tag the Docker image
        sh 'docker build -t siddharth67/solar-system:$GIT_COMMIT .'
    }
}
GIT_COMMIT is a built-in Jenkins variable provided when you use the checkout step. It resolves to the current commit SHA, ensuring each Docker image is uniquely tagged.

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Jenkinsfile Reference

For more information on pipeline syntax and environment variables, see the Jenkins Pipeline Syntax documentation.

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Dockerfile Breakdown

Your repository includes this Dockerfile, which defines how to build the Node.js application image. 
FROM node:18-alpine3.17

WORKDIR /usr/app

COPY package*.json /usr/app/
RUN npm install

COPY . .

# Placeholder environment variables for MongoDB credentials
ENV MONGO_URI=uriPlaceholder
ENV MONGO_USERNAME=usernamePlaceholder
ENV MONGO_PASSWORD=passwordPlaceholder

EXPOSE 3000
CMD [ "npm", "start" ]
Key steps:
  1. FROM: Start from the lightweight node:18-alpine3.17 base image.
  2. WORKDIR: Set /usr/app as the working directory.
  3. COPY package*.json: Copy dependency manifests and install packages.
  4. COPY . .: Add application source code.
  5. ENV: Define placeholders for MongoDB connection.
  6. EXPOSE: Open port 3000.
  7. CMD: Launch the app with npm start.
For complete details, refer to the Dockerfile reference.

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.dockerignore

Optimize build context by excluding unnecessary files. Place this .dockerignore in the project root: 
# Source control and metadata
.git
.github/

# Node.js dependencies
node_modules

# Configuration and reports
.*  
!.README*.md
README-secret.md
solar-system.png
.nyc_output
.talismanrc
coverage
test-results.xml
reports*
zap*
dependency*
jenkins*
trivy-image*
Be cautious when excluding files. Make sure you don’t accidentally omit critical configuration, scripts, or assets required at runtime.
See the Dockerignore reference for patterns and best practices.

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Jenkins Pipeline Environment Variables

Jenkins exposes numerous environment variables in multibranch pipelines. Visit Pipeline Syntax → Global variables reference to explore them all.
VariableDescription
GIT_COMMITCurrent commit SHA (requires checkout)
BRANCH_NAMEActive branch in a multibranch pipeline
CHANGE_IDPull request or change request identifier
BUILD_NUMBERSequential build number
BUILD_IDUnique build identifier
WORKSPACEPath to the workspace on the agent
NODE_NAMEName of the Jenkins agent node running the build
The image shows a Jenkins Pipeline Syntax page detailing environment variables available for multibranch projects, such as BRANCH_NAME, CHANGE_ID, and CHANGE_AUTHOR.

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Pipeline Execution

After committing and pushing the updated Jenkinsfile, Jenkins triggers a new run. Once earlier stages finish, the Build Docker Image stage will begin: 
docker build -t siddharth67/solar-system:$GIT_COMMIT .
This step logs all environment variables visible to the build stage: 
+ printenv
JENKINS_HOME=/var/lib/jenkins
GIT_PREVIOUS_SUCCESSFUL_COMMIT=10f241dbfe4218e2d9acd44b9950c4144
MONGO_DB_CREDS_PSW=****
USER=jenkins
CI=true
…  
GIT_COMMIT=0bb4c412562f4f1db4c2149f834e29f3
BUILD_URL=http://jenkins.example.com/job/.../28/
WORKSPACE=/var/lib/jenkins/workspace/solar-system_feature_enabling-cicd
STAGE_NAME=Build Docker Image
GIT_BRANCH=feature/enabling-cicd
BUILD_TAG=jenkins-Gitea-Organization-solar-system-feature%252Fenabling-cicd-28

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Docker Build Logs

Jenkins streams the Docker build output, showing layer creation and tagging: 
#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 2828B done
…
#9 naming to docker.io/siddharth67/solar-system:9dbc4b421562f410b4dec2141938fd2a5ac0ad1 done
#9 DONE 0.0s
Verify the image tag in your registry or local Docker daemon to ensure it matches the commit SHA.
The image shows a code repository interface with a list of files and recent commits, including a branch named "feature/enabling-cicd."

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Next Steps

You’ve successfully built and tagged a Docker image via Jenkins. In the next tutorial, we’ll scan this image for vulnerabilities and push it to Docker Hub.

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