Docker image as artifact in Jenkins - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Jenkins pipeline snippet?

Consider this Jenkins pipeline step that builds a Docker image and tags it:

stage('Build Docker Image') {
  steps {
    script {
      dockerImage = docker.build("myapp:${env.BUILD_NUMBER}")
      echo dockerImage.id
    }
  }
}

What will be printed by echo dockerImage.id?

Jenkins

stage('Build Docker Image') {
  steps {
    script {
      dockerImage = docker.build("myapp:${env.BUILD_NUMBER}")
      echo dockerImage.id
    }
  }
}

A An error because dockerImage.id is not defined

B The string 'myapp:${env.BUILD_NUMBER}' literally

C The Jenkins build number only

D The unique image ID hash of the built Docker image

💡 Hint

Think about what docker.build() returns and what id represents.

The docker.build() method returns a Docker image object. The id property is the unique hash ID of the built image, which Jenkins can print.

🔀 Workflow

intermediate

Which Jenkins pipeline step correctly pushes a Docker image as an artifact to Docker Hub?

You have built a Docker image in Jenkins with tag myapp:${env.BUILD_NUMBER}. Which step correctly pushes this image to Docker Hub?

A sh 'docker push myapp:${env.BUILD_NUMBER}'

B docker.withRegistry('https://registry.hub.docker.com', 'dockerhub-credentials') { dockerImage.push() }

C docker.push('myapp:${env.BUILD_NUMBER}')

D dockerImage.push('latest')

💡 Hint

Remember Jenkins needs to authenticate to Docker Hub before pushing.

docker.withRegistry() wraps the push command with authentication. Calling dockerImage.push() inside it pushes the tagged image correctly.

❓ Troubleshoot

advanced

Why does this Jenkins pipeline fail to push the Docker image?

Given this snippet:

stage('Push Image') {
  steps {
    script {
      dockerImage = docker.build("myapp:${env.BUILD_NUMBER}")
      dockerImage.push()
    }
  }
}

The pipeline fails with an authentication error. What is the most likely cause?

A Missing <code>docker.withRegistry</code> block to provide Docker Hub credentials

B The image tag is invalid and causes push failure

C The <code>docker.build</code> command did not complete successfully

D The Jenkins agent does not have Docker installed

💡 Hint

Think about how Jenkins authenticates to Docker registries.

Without docker.withRegistry and credentials, Jenkins cannot authenticate to Docker Hub, causing push to fail.

🧠 Conceptual

advanced

What is the main benefit of using Docker images as artifacts in Jenkins pipelines?

Why do DevOps teams prefer storing Docker images as build artifacts in Jenkins pipelines?

A It ensures consistent deployment by using the exact same image built and tested

B It reduces the size of the Jenkins workspace by deleting source code

C It speeds up the Jenkins build by skipping tests

D It allows Jenkins to run containers without Docker installed on agents

💡 Hint

Think about consistency and reliability in deployments.

Storing Docker images as artifacts ensures the same tested image is deployed, avoiding differences between build and production.

✅ Best Practice

expert

Which Jenkinsfile snippet follows best practices for building, tagging, and pushing a Docker image as an artifact?

Choose the Jenkins pipeline snippet that correctly builds a Docker image, tags it with the build number and 'latest', pushes both tags to Docker Hub, and uses credentials securely.

A

stage('Build and Push') {
  steps {
    script {
      dockerImage = docker.build("myapp:${env.BUILD_NUMBER}")
      dockerImage.push('latest')
      dockerImage.push()
    }
  }
}

B

stage('Build and Push') {
  steps {
    sh 'docker build -t myapp:${env.BUILD_NUMBER} .'
    sh 'docker push myapp:${env.BUILD_NUMBER}'
    sh 'docker push myapp:latest'
  }
}

C

stage('Build and Push') {
  steps {
    script {
      dockerImage = docker.build("myapp:${env.BUILD_NUMBER}")
      docker.withRegistry('https://registry.hub.docker.com', 'dockerhub-credentials') {
        dockerImage.push()
        dockerImage.push('latest')
      }
    }
  }
}

D

stage('Build and Push') {
  steps {
    script {
      dockerImage = docker.build("myapp:latest")
      dockerImage.push()
      dockerImage.push('${env.BUILD_NUMBER}')
    }
  }
}

💡 Hint

Look for correct tag usage, push order, and secure credential handling.

Option C builds with the build number tag, pushes that tag and 'latest' inside a docker.withRegistry block using credentials. This is the recommended secure and clear approach.

Practice

1. What is the main reason to save a Docker image as an artifact in Jenkins?

easy

A. To run the image on a different operating system

B. To delete the image after the build

C. To convert the image into a container

D. To reuse the image later without rebuilding it

Solution

1. Step 1: Understand Docker image reuse

   Saving a Docker image as an artifact allows you to reuse it later without rebuilding, saving time and resources.

2. Step 2: Identify Jenkins artifact purpose

   Jenkins artifacts store files from builds for later use or sharing, perfect for Docker images.

3. Final Answer:

   To reuse the image later without rebuilding it -> Option D

4. Quick Check:

   Docker image reuse = To reuse the image later without rebuilding it [OK]

Hint: Artifacts store files to reuse later, like saving your work [OK]

Common Mistakes:

• Thinking artifacts delete images
• Confusing image with container
• Assuming artifacts change OS compatibility

2. Which Jenkins pipeline step correctly archives a saved Docker image file named myimage.tar?

easy

A. archiveArtifacts artifacts: 'myimage.tar'

B. archiveArtifacts file: 'myimage.tar'

C. archiveArtifact artifacts: 'myimage.tar'

D. archiveArtifacts files: 'myimage.tar'

Solution

1. Step 1: Recall Jenkins archiveArtifacts syntax

   The correct syntax uses the parameter name 'artifacts' to specify files to archive.

2. Step 2: Match correct parameter and spelling

   archiveArtifacts artifacts: 'myimage.tar' uses 'archiveArtifacts artifacts: 'myimage.tar'', which is correct syntax.

3. Final Answer:

   archiveArtifacts artifacts: 'myimage.tar' -> Option A

4. Quick Check:

   Correct parameter is 'artifacts' = archiveArtifacts artifacts: 'myimage.tar' [OK]

Hint: Use 'artifacts' parameter exactly in archiveArtifacts step [OK]

Common Mistakes:

• Using 'file' instead of 'artifacts'
• Misspelling 'archiveArtifacts'
• Using singular 'archiveArtifact'

3. Given this Jenkins pipeline snippet:

stage('Save Image') {
  steps {
    sh 'docker save myapp:latest -o myapp.tar'
    archiveArtifacts artifacts: 'myapp.tar'
  }
}

What will Jenkins do after this stage completes?

medium

A. Delete the Docker image myapp:latest

B. Save the Docker image as a file and archive it as a build artifact

C. Run the Docker container from the saved image

D. Build a new Docker image named myapp:latest

Solution

1. Step 1: Analyze docker save command

   The command 'docker save myapp:latest -o myapp.tar' saves the Docker image to a file named myapp.tar.

2. Step 2: Understand archiveArtifacts usage

   The 'archiveArtifacts' step archives the saved file as a Jenkins build artifact for later use.

3. Final Answer:

   Save the Docker image as a file and archive it as a build artifact -> Option B

4. Quick Check:

   docker save + archiveArtifacts = Save the Docker image as a file and archive it as a build artifact [OK]

Hint: docker save creates file; archiveArtifacts stores it [OK]

Common Mistakes:

• Confusing save with build
• Thinking it runs containers
• Assuming image is deleted

4. You wrote this Jenkins pipeline snippet:

sh 'docker save myapp:latest -o myapp.tar'
archiveArtifacts file: 'myapp.tar'

Why does Jenkins fail to archive the Docker image file?

medium

A. Because 'docker save' does not create a file

B. Because the image name is incorrect

C. Because the parameter name should be 'artifacts', not 'file'

D. Because archiveArtifacts cannot archive .tar files

Solution

1. Step 1: Check archiveArtifacts parameter

   The correct parameter to specify files is 'artifacts', not 'file'. Using 'file' causes failure.

2. Step 2: Confirm docker save output

   'docker save' correctly creates the file 'myapp.tar', so the problem is not with the file creation.

3. Final Answer:

   Because the parameter name should be 'artifacts', not 'file' -> Option C

4. Quick Check:

   Parameter must be 'artifacts' = Because the parameter name should be 'artifacts', not 'file' [OK]

Hint: Use 'artifacts' parameter exactly in archiveArtifacts step [OK]

Common Mistakes:

• Using 'file' instead of 'artifacts'
• Assuming docker save fails silently
• Thinking archiveArtifacts rejects .tar files

5. You want to reuse a Docker image saved as appimage.tar from a previous Jenkins build artifact in a new pipeline. Which sequence correctly loads and runs the image?

hard

A. sh 'docker load -i appimage.tar'
sh 'docker run appimage:latest'

B. sh 'docker run -i appimage.tar'
sh 'docker load appimage:latest'

C. sh 'docker save -i appimage.tar'
sh 'docker run appimage:latest'

D. sh 'docker load appimage.tar'
sh 'docker run appimage:latest'

Solution

1. Step 1: Understand docker load syntax

   To load an image from a file, use 'docker load -i filename'. This imports the image into Docker.

2. Step 2: Run the loaded image

   After loading, run the image by its tag, e.g., 'docker run appimage:latest'.

3. Final Answer:

   sh 'docker load -i appimage.tar' sh 'docker run appimage:latest' -> Option A

4. Quick Check:

   Load with '-i', then run by tag = sh 'docker load -i appimage.tar' sh 'docker run appimage:latest' [OK]

Hint: Load with 'docker load -i', then run image by tag [OK]

Common Mistakes:

• Using 'docker run' with file instead of image tag
• Confusing 'docker save' with 'docker load'
• Omitting '-i' option in docker load