Pushing images to registry in Jenkins - Mini Project: Build & Apply

Pushing images to registry

📖 Scenario: You are working as a DevOps engineer. Your team uses Jenkins to automate building and pushing Docker images to a container registry. You want to create a simple Jenkins pipeline script that builds a Docker image from a Dockerfile and pushes it to a registry.

🎯 Goal: Build a Jenkins pipeline script that builds a Docker image named myapp with tag v1 and pushes it to the Docker registry at docker.io/myuser/myapp:v1.

📋 What You'll Learn

Create a variable for the Docker image name

Create a variable for the Docker image tag

Use the docker.build command to build the image

Use the docker.withRegistry block to push the image

Print a message after pushing the image

💡 Why This Matters

🌍 Real World

Automating Docker image builds and pushes is common in continuous integration pipelines to ensure consistent deployments.

💼 Career

DevOps engineers and automation specialists use Jenkins pipelines to manage container images and deploy applications efficiently.

1

Set Docker image name and tag

Create two variables in the Jenkins pipeline script: imageName set to myapp and imageTag set to v1.

Jenkins

# Create variables imageName and imageTag
# Your code here

Hint

Use def to declare variables in Jenkins scripted pipeline.

2

Build the Docker image

Use the docker.build command to build the Docker image with the name "${imageName}:${imageTag}" and assign it to a variable called appImage.

Jenkins

def imageName = 'myapp'
def imageTag = 'v1'
# Build the Docker image and assign to appImage
# Your code here

Hint

Use docker.build with the image name and tag inside double quotes and curly braces.

3

Push the Docker image to registry

Use docker.withRegistry with the URL 'https://docker.io' and credentials ID 'dockerhub-creds' to push the image stored in appImage.

Jenkins

def imageName = 'myapp'
def imageTag = 'v1'
def appImage = docker.build("${imageName}:${imageTag}")
# Push the image to Docker registry
# Your code here

Hint

Wrap the push command inside docker.withRegistry block with the registry URL and credentials ID.

4

Print confirmation message

Add a println statement to print "Docker image pushed: myapp:v1" after the image is pushed.

Jenkins

def imageName = 'myapp'
def imageTag = 'v1'
def appImage = docker.build("${imageName}:${imageTag}")
docker.withRegistry('https://docker.io', 'dockerhub-creds') {
    appImage.push()
}
# Print confirmation message
# Your code here

Hint

Use println with the exact message including image name and tag.

Practice

1. What is the main purpose of pushing a Docker image to a registry in Jenkins?

easy

A. To run the Docker image on the Jenkins server

B. To delete the local Docker image after building

C. To create a new Dockerfile automatically

D. To save the built image so it can be shared and deployed elsewhere

Solution

1. Step 1: Understand Docker image pushing

   Pushing an image uploads it to a registry so others can access it.

2. Step 2: Connect to Jenkins usage

   Jenkins automates this to share images for deployment or other jobs.

3. Final Answer:

   To save the built image so it can be shared and deployed elsewhere -> Option D

4. Quick Check:

   Pushing = Save and share image [OK]

Hint: Pushing means uploading image for sharing [OK]

Common Mistakes:

• Confusing push with running the image
• Thinking push deletes local images
• Assuming push creates Dockerfiles

2. Which Jenkins pipeline step correctly pushes a Docker image named myapp:latest to Docker Hub after login?

easy

A. sh 'docker push myapp:latest'

B. sh 'docker pull myapp:latest'

C. sh 'docker build myapp:latest'

D. sh 'docker run myapp:latest'

Solution

1. Step 1: Identify the push command

   To upload an image, the command is docker push followed by the image name and tag.

2. Step 2: Check other commands

   docker pull downloads, docker build creates, and docker run executes images.

3. Final Answer:

   sh 'docker push myapp:latest' -> Option A

4. Quick Check:

   Push uploads image [OK]

Hint: Push uses 'docker push' command [OK]

Common Mistakes:

• Using pull instead of push
• Confusing build with push
• Trying to run instead of push

3. Given this Jenkins pipeline snippet, what will be the output if the Docker login fails?

stage('Push Image') {
  steps {
    sh 'docker login -u user -p wrongpassword'
    sh 'docker push myapp:latest'
  }
}

medium

A. The image will be pushed anonymously

B. The push will succeed anyway

C. The push will fail because login failed

D. The pipeline will skip the push step

Solution

1. Step 1: Understand docker login failure effect

   If login fails, Docker does not authenticate, so push cannot succeed to a private registry.

2. Step 2: Check pipeline behavior

   Since login command fails, the push command will also fail due to lack of permissions.

3. Final Answer:

   The push will fail because login failed -> Option C

4. Quick Check:

   Failed login blocks push [OK]

Hint: Push needs successful login first [OK]

Common Mistakes:

• Assuming push works without login
• Thinking pipeline skips failed steps automatically
• Believing anonymous push is allowed

4. You have this Jenkins pipeline snippet but the image is not pushed. What is the error?

stage('Push') {
  steps {
    sh 'docker login -u user -p $PASSWORD'
    sh 'docker push myapp'
  }
}

medium

A. The login command syntax is incorrect

B. The image tag is missing in the push command

C. The password variable should be in double quotes

D. Docker push does not require login

Solution

1. Step 1: Check the push command syntax

   The push command must specify the image name with a tag, e.g., myapp:latest. Without a tag, it defaults to 'latest' but explicit is better.

2. Step 2: Verify other parts

   Login syntax is correct, password variable usage is valid in shell, and login is required before push.

3. Final Answer:

   The image tag is missing in the push command -> Option B

4. Quick Check:

   Push needs image:tag [OK]

Hint: Always specify image tag when pushing [OK]

Common Mistakes:

• Omitting image tag causes push issues
• Misusing environment variables in shell
• Skipping login before push

5. In a Jenkins pipeline, you want to push a Docker image to a private registry that requires login. Which sequence correctly automates this process?

hard

A. Login with credentials, tag image with registry URL, then push tagged image

B. Push image first, then login to registry, then tag image

C. Tag image without login, then push, then login after push

D. Login, push image without tagging, then tag image

Solution

1. Step 1: Understand Docker push workflow

   You must login first to authenticate, then tag the image with the registry URL to specify destination, then push the tagged image.

2. Step 2: Analyze options

   Options B, C, and D have wrong order causing push failures or wrong image pushed.

3. Final Answer:

   Login with credentials, tag image with registry URL, then push tagged image -> Option A

4. Quick Check:

   Login -> tag -> push [OK]

Hint: Login first, tag image, then push [OK]

Common Mistakes:

• Pushing before login
• Pushing untagged images to private registry
• Tagging after pushing