Docker Pipeline plugin in Jenkins - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker Pipeline build step

What is the output of the following Jenkins Pipeline snippet using the Docker Pipeline plugin?

Jenkins

pipeline {
  agent any
  stages {
    stage('Build') {
      steps {
        script {
          def img = docker.build('my-image:latest')
          echo img.id
        }
      }
    }
  }
}

A The image ID string of the built Docker image, e.g., sha256:abc123...

B The literal string 'my-image:latest'

C An error: 'docker.build' is not a valid method

D The container ID of a running container

💡 Hint

The docker.build() method returns an image object with an id property.

The docker.build('my-image:latest') command builds the Docker image and returns an image object. The img.id property contains the image's unique ID string.

🧠 Conceptual

intermediate

Understanding Docker Pipeline plugin's 'inside' method

In Jenkins Docker Pipeline plugin, what does the 'inside' method do when called on a Docker image object?

A Pushes the Docker image to a remote registry.

B Builds a new Docker image from the current workspace.

C Runs the given block of pipeline steps inside a container created from the image.

D Removes the Docker image from the local Docker host.

💡 Hint

Think about running commands inside a container environment.

The 'inside' method runs the enclosed pipeline steps inside a container started from the specified Docker image, providing an isolated environment.

🔀 Workflow

advanced

Correct order of Docker Pipeline plugin steps to build, test, and push

Arrange the following Docker Pipeline plugin steps in the correct order to build an image, run tests inside it, and then push it to a registry.

A 1,4,2,3

B 1,2,4,3

C 4,2,1,3

D 4,1,2,3

💡 Hint

You must define the image variable before using it.

First, define the image variable by assigning the result of docker.build. Then build the image, run tests inside it, and finally push it.

❓ Troubleshoot

advanced

Diagnosing Docker Pipeline plugin authentication failure

You get an error 'unauthorized: authentication required' when running 'image.push()' in Jenkins Docker Pipeline plugin. What is the most likely cause?

A The Dockerfile has syntax errors.

B Docker registry credentials are not configured or not passed to the pipeline.

C The Jenkins agent does not have Docker installed.

D The Docker image was not built before pushing.

💡 Hint

Pushing requires authentication to the registry.

The 'unauthorized' error means the pipeline lacks proper credentials to push to the Docker registry.

✅ Best Practice

expert

Best practice for cleaning up Docker images in Jenkins pipeline

Which approach is best to avoid disk space issues caused by leftover Docker images when using the Docker Pipeline plugin in Jenkins?

A Use 'docker.image(...).remove()' after pushing the image to clean up local images.

B Manually delete images on the Jenkins agent outside the pipeline.

C Do not build images in the pipeline; use pre-built images only.

D Restart the Jenkins server daily to clear Docker cache.

💡 Hint

Cleaning up images programmatically after use is efficient.

Calling 'remove()' on the image object after pushing deletes the local image and frees disk space automatically.