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Learn how to build a custom Docker image via a Dockerfile and use it as an agent in a Jenkins Pipeline stage. This method lets you pre-install all required tools—like Node.js and cowsay—so your CI/CD steps run smoothly.

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1. Using the Standard node:18-alpine Docker Agent

First, try running Node.js commands and the cowsay utility using the official node:18-alpine image:
Commit and push this Jenkinsfile, then trigger a build.
The image shows a Jenkins dashboard displaying the activity of a pipeline named "pipeline-external-agent," with details of recent runs, their status, duration, and completion times.
The S4-Dockerfile Agent stage will fail because cowsay is not installed in the base image.

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2. Diagnosing the Missing Utility

Inspect the console output to confirm which utility is missing:
  • Node.js commands succeed (node -v, npm -v)
  • cowsay fails: the utility isn’t present in node:18-alpine.

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3. Crafting a Custom Dockerfile

Create Dockerfile.cowsay at your repository root to bundle Node.js and cowsay:
This Dockerfile:
  1. Base Image: node:18-alpine
  2. Dependencies: Installs git and perl via Alpine’s package manager
  3. Cowsay: Clones the cowsay GitHub repository and runs its installer
Using Alpine keeps the image slim. Ensure you list all required packages in the RUN apk add command.

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4. Updating Your Jenkinsfile to Use the Custom Image

Swap the docker agent block for a dockerfile block:
Commit and push. Jenkins will now build your custom image before executing the stage.

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5. Observing the Build Output

When you trigger the build, you’ll see:
Once the image builds, Jenkins runs your container and executes the steps:
Now both Node.js and cowsay run successfully inside the container.

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6. Agent Types Comparison

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7. Conclusion

By leveraging a custom Dockerfile agent, you ensure each Jenkins stage has exactly the tools it needs. This strategy:
  • Simplifies dependency management
  • Keeps agents lightweight
  • Improves reproducibility

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