Docker Pipeline plugin in Jenkins - Mini Project: Build & Apply

Build and Push Docker Image Using Jenkins Docker Pipeline Plugin

📖 Scenario: You are working as a DevOps engineer. Your team wants to automate building and pushing Docker images using Jenkins. You will create a Jenkins pipeline script that uses the Docker Pipeline plugin to build a Docker image from a Dockerfile and push it to Docker Hub.

🎯 Goal: Build a Jenkins pipeline script that builds a Docker image named myapp with tag v1 from the current directory, then pushes it to Docker Hub repository yourdockerhubid/myapp:v1.

📋 What You'll Learn

Create a Docker image named myapp with tag v1 using the Docker Pipeline plugin

Use a variable dockerImage to hold the image object

Push the image to Docker Hub repository yourdockerhubid/myapp:v1

Print a message confirming the image push

💡 Why This Matters

🌍 Real World

Automating Docker image builds and pushes in Jenkins pipelines is common in continuous integration and continuous delivery (CI/CD) workflows.

💼 Career

DevOps engineers and automation specialists use Docker Pipeline plugin in Jenkins to streamline container image management and deployment.

1

Create Docker image variable

Create a variable called dockerImage that builds a Docker image named myapp:v1 from the current directory using docker.build.

Jenkins

# Your code here

Hint

Use docker.build('myapp:v1') to build the image and assign it to dockerImage.

2

Add Docker Hub repository variable

Create a variable called dockerRepo and set it to the string 'yourdockerhubid/myapp:v1' to specify the Docker Hub repository and tag.

Jenkins

dockerImage = docker.build('myapp:v1')
# Your code here

Hint

Assign the exact string 'yourdockerhubid/myapp:v1' to dockerRepo.

3

Tag and push Docker image

Use the dockerImage variable to tag the image with dockerRepo and push it to Docker Hub using dockerImage.push().

Jenkins

dockerImage = docker.build('myapp:v1')
dockerRepo = 'yourdockerhubid/myapp:v1'
# Your code here

Hint

Use dockerImage.tag(dockerRepo) to tag and dockerImage.push() to push the image.

4

Print confirmation message

Print the message "Docker image pushed to yourdockerhubid/myapp:v1" using echo.

Jenkins

dockerImage = docker.build('myapp:v1')
dockerRepo = 'yourdockerhubid/myapp:v1'
dockerImage.tag(dockerRepo)
dockerImage.push()
# Your code here

Hint

Use echo "Docker image pushed to yourdockerhubid/myapp:v1" to print the confirmation.

Practice

1. What is the primary purpose of the docker.build() method in Jenkins Docker Pipeline plugin?

easy

A. To remove unused Docker images

B. To run a Docker container interactively

C. To build a Docker image from a Dockerfile

D. To push a Docker image to a registry

Solution

1. Step 1: Understand the role of docker.build()

   This method is used to create a Docker image from a Dockerfile in the workspace.

2. Step 2: Differentiate from other Docker Pipeline methods

   Running containers uses docker.image().inside { }, pushing uses docker.withRegistry(), and removing images is not handled by docker.build().

3. Final Answer:

   To build a Docker image from a Dockerfile -> Option C

4. Quick Check:

   docker.build() builds images [OK]

Hint: Remember: build = create image from Dockerfile [OK]

Common Mistakes:

• Confusing build with push or run commands
• Thinking docker.build() runs containers
• Assuming it handles image removal

2. Which of the following is the correct syntax to run commands inside a Docker container in a Jenkins pipeline?

easy

A. docker.image('myimage').inside { sh 'echo Hello' }

B. docker.build('myimage') { sh 'echo Hello' }

C. docker.run('myimage') { sh 'echo Hello' }

D. docker.container('myimage').exec { sh 'echo Hello' }

Solution

1. Step 1: Identify the correct method to run commands inside a container

   The docker.image('myimage').inside { } syntax runs commands inside a container created from the image.

2. Step 2: Check other options for syntax errors or misuse

   docker.run() and docker.container() are not valid Jenkins Pipeline methods; docker.build() builds images but does not run containers.

3. Final Answer:

   docker.image('myimage').inside { sh 'echo Hello' } -> Option A

4. Quick Check:

   Use docker.image().inside { } to run commands [OK]

Hint: Use docker.image().inside { } to run commands inside containers [OK]

Common Mistakes:

• Using docker.build() to run commands
• Trying to use non-existent methods like docker.run()
• Confusing image build and container run syntax

3. Given the following Jenkins pipeline snippet, what will be the output of the sh command inside the container?

def img = docker.build('myapp:latest')
img.inside {
  sh 'echo Hello from Docker'
}

medium

A. Error: No such image 'myapp:latest'

B. Hello from Docker

C. Build logs of the Docker image

D. Nothing, the command is not executed

Solution

1. Step 1: Understand the build and run sequence

   The docker.build('myapp:latest') creates the image, then img.inside { } runs commands inside a container from that image.

2. Step 2: Analyze the sh command inside the container

   The shell command echo Hello from Docker runs inside the container and outputs the string.

3. Final Answer:

   Hello from Docker -> Option B

4. Quick Check:

   Commands inside img.inside { } run and output normally [OK]

Hint: Commands inside img.inside { } run inside container [OK]

Common Mistakes:

• Assuming build only creates logs, not image
• Thinking the command runs outside the container
• Expecting an error if image was just built

4. Identify the error in this Jenkins pipeline snippet using Docker Pipeline plugin:

docker.withRegistry('https://myregistry.com', 'my-credentials') {
  def img = docker.build('myapp')
  img.push()
}
img.inside {
  sh 'echo Running inside container'
}

medium

A. The img.push() should be called before docker.withRegistry

B. The docker.build method is missing the Dockerfile path

C. The credentials ID 'my-credentials' is invalid syntax

D. The img.inside block is outside the docker.withRegistry block and will fail

Solution

1. Step 1: Check scope of img variable

   The variable img is defined inside the docker.withRegistry block, so it is not accessible outside that block.

2. Step 2: Identify the error with img.inside usage

   Using img.inside outside the block where img is defined causes a runtime error because img is undefined there.

3. Final Answer:

   The img.inside block is outside the docker.withRegistry block and will fail -> Option D

4. Quick Check:

   Variable scope matters in Jenkins pipeline blocks [OK]

Hint: Keep variable usage inside the block where it's defined [OK]

Common Mistakes:

• Assuming variables are global across blocks
• Confusing credentials ID syntax
• Thinking push must be outside withRegistry

5. You want to build a Docker image, run tests inside a container from that image, and then push the image to a private registry using Jenkins Docker Pipeline plugin. Which pipeline snippet correctly implements this workflow?

hard

A. def img = docker.build('myapp:test')
img.inside {
sh 'run-tests.sh'
}
docker.withRegistry('https://registry.example.com', 'creds') {
img.push()
}

B. docker.withRegistry('https://registry.example.com', 'creds') {
def img = docker.build('myapp:test')
img.push()
img.inside {
sh 'run-tests.sh'
}
}

C. docker.withRegistry('https://registry.example.com', 'creds') {
img.push()
def img = docker.build('myapp:test')
img.inside {
sh 'run-tests.sh'
}
}

D. def img = docker.build('myapp:test')
docker.withRegistry('https://registry.example.com', 'creds') {
img.push()
}
img.inside {
sh 'run-tests.sh'
}

Solution

1. Step 1: Build image and run tests inside container first

   The image must be built before running tests inside it, so docker.build() and img.inside { } come first.

2. Step 2: Push image inside docker.withRegistry() block after tests

   Pushing requires registry credentials, so img.push() must be inside docker.withRegistry() block after tests pass.

3. Step 3: Verify variable scope and order

   def img = docker.build('myapp:test') img.inside { sh 'run-tests.sh' } docker.withRegistry('https://registry.example.com', 'creds') { img.push() } correctly builds image, runs tests inside container, then pushes image with credentials in proper order and scope.

4. Final Answer:

   def img = docker.build('myapp:test') img.inside { sh 'run-tests.sh' } docker.withRegistry('https://registry.example.com', 'creds') { img.push() } -> Option A

5. Quick Check:

   Build -> Test inside container -> Push with credentials [OK]

Hint: Build first, test inside container, then push inside withRegistry [OK]

Common Mistakes:

• Pushing image before running tests
• Using img.inside after pushing outside scope
• Defining variables inside wrong blocks