Pushing images from CI in Docker - Mini Project: Build & Apply

Pushing Docker Images from CI

📖 Scenario: You work in a team that builds Docker images for your app. You want to automate pushing these images to Docker Hub using a Continuous Integration (CI) pipeline.This project guides you to write a simple Dockerfile, set up image tagging, and push the image to Docker Hub from a CI script.

🎯 Goal: Build a Docker image from a Dockerfile, tag it with a version, and push it to Docker Hub using a CI script.

📋 What You'll Learn

Create a Dockerfile with a simple base image

Set a variable for the image tag

Write a command to build and tag the Docker image

Write a command to push the tagged image to Docker Hub

💡 Why This Matters

🌍 Real World

Automating Docker image builds and pushes saves time and avoids manual errors in software delivery.

💼 Career

CI pipelines that build and push Docker images are common tasks for DevOps engineers and developers working with containerized apps.

1

Create a simple Dockerfile

Create a file named Dockerfile with these exact contents:
FROM alpine:3.18
CMD ["echo", "Hello from Docker"]

Docker

# Create a Dockerfile with FROM alpine:3.18 and CMD ["echo", "Hello from Docker"]

Hint

This Dockerfile uses Alpine Linux as the base and prints a message when run.

2

Set the image tag variable

In a file named ci_script.sh, create a variable called IMAGE_TAG and set it to "myapp:1.0".

Docker

FROM alpine:3.18
CMD ["echo", "Hello from Docker"]

# Set IMAGE_TAG variable
# Your code here

Hint

Use IMAGE_TAG="myapp:1.0" exactly to set the tag.

3

Build and tag the Docker image

In ci_script.sh, add a command to build the Docker image from the current directory and tag it with $IMAGE_TAG.

Docker

FROM alpine:3.18
CMD ["echo", "Hello from Docker"]

IMAGE_TAG="myapp:1.0"

# Build and tag the Docker image
# Your code here

Hint

Use docker build -t $IMAGE_TAG . to build and tag the image.

4

Push the Docker image to Docker Hub

In ci_script.sh, add a command to push the Docker image tagged with $IMAGE_TAG to Docker Hub.

Docker

FROM alpine:3.18
CMD ["echo", "Hello from Docker"]

IMAGE_TAG="myapp:1.0"

docker build -t $IMAGE_TAG .

# Push the Docker image
# Your code here

Hint

Use docker push $IMAGE_TAG to push the image.

Practice

1. What is the main purpose of pushing Docker images from a CI pipeline?

easy

A. To delete old images from the local Docker cache

B. To run containers automatically on the local machine

C. To upload built images to a remote registry for sharing and deployment

D. To build images only on the developer's computer

Solution

1. Step 1: Understand CI pipeline role

   A CI pipeline automates building and testing code changes, including Docker images.

2. Step 2: Purpose of pushing images

   Pushing images uploads them to a remote registry so others can use or deploy them.

3. Final Answer:

   To upload built images to a remote registry for sharing and deployment -> Option C

4. Quick Check:

   Pushing images = Upload to registry [OK]

Hint: Pushing means sending images to a remote place [OK]

Common Mistakes:

• Confusing pushing with running containers
• Thinking pushing deletes images
• Assuming pushing only builds images locally

2. Which command correctly pushes a Docker image named myapp:latest to Docker Hub in a CI script?

easy

A. docker publish myapp:latest

B. docker upload myapp:latest

C. docker send myapp:latest

D. docker push myapp:latest

Solution

1. Step 1: Identify Docker push syntax

   The correct command to upload images to a registry is docker push <image>.

2. Step 2: Verify other options

   Commands like upload, send, and publish are not valid Docker commands.

3. Final Answer:

   docker push myapp:latest -> Option D

4. Quick Check:

   Push = docker push [OK]

Hint: Use 'docker push' to send images to registry [OK]

Common Mistakes:

• Using 'docker upload' instead of 'docker push'
• Confusing push with publish or send commands
• Missing image tag in push command

3. Given this CI script snippet:

docker build -t myuser/myapp:1.0 .
docker login -u myuser -p $DOCKER_PASS
docker push myuser/myapp:1.0

What will happen if $DOCKER_PASS is empty?

medium

A. The push will fail due to authentication error

B. The image will push successfully without login

C. The build command will fail before push

D. The image will be pushed anonymously

Solution

1. Step 1: Understand docker login with empty password

   If $DOCKER_PASS is empty, login will fail because no valid password is provided.

2. Step 2: Effect on docker push

   Without successful login, pushing to Docker Hub requires authentication and will fail.

3. Final Answer:

   The push will fail due to authentication error -> Option A

4. Quick Check:

   Empty password = login fail = push fail [OK]

Hint: Empty password means login fails, so push fails [OK]

Common Mistakes:

• Assuming push works without login
• Thinking build fails due to empty password
• Believing anonymous push is allowed

4. You have this CI step:

docker push myuser/myapp:latest

But the push fails with denied: requested access to the resource is denied. What is the most likely cause?

medium

A. The image tag latest is invalid

B. You forgot to run docker login before pushing

C. The Dockerfile has syntax errors

D. The Docker daemon is not running

Solution

1. Step 1: Analyze error message meaning

   The error means Docker Hub denied access, usually due to missing or wrong authentication.

2. Step 2: Check common causes

   Not running docker login before push is the most common cause of access denial.

3. Final Answer:

   You forgot to run docker login before pushing -> Option B

4. Quick Check:

   Access denied = missing login [OK]

Hint: Always login before pushing images [OK]

Common Mistakes:

• Blaming image tag for access denied
• Assuming Dockerfile errors cause push failure
• Ignoring Docker daemon status

5. In a CI pipeline, you want to push a Docker image only if the build and tests succeed. Which is the best way to ensure this?

hard

A. Add a separate job that depends on build and test jobs and pushes the image

B. Push the image immediately after the build step without waiting for tests

C. Push the image before running tests to save time

D. Push the image manually after the pipeline finishes

Solution

1. Step 1: Understand CI job dependencies

   To push only after build and tests succeed, create a job that runs after those jobs complete successfully.

2. Step 2: Why other options fail

   Pushing before tests risks pushing broken images; manual push loses automation benefits.

3. Final Answer:

   Add a separate job that depends on build and test jobs and pushes the image -> Option A

4. Quick Check:

   Push after success = dependent job [OK]

Hint: Use dependent jobs to control push timing [OK]

Common Mistakes:

• Pushing before tests complete
• Manual push breaks automation
• Ignoring job dependencies in CI