Pushing images from CI in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

To automatically upload built Docker images to a container registry so they can be used in deployment or shared with others.

beginner

Which command is used to push a Docker image to a registry?

The command is docker push <image-name>. It uploads the image to the specified registry.

beginner

Why do you need to log in to a Docker registry in your CI pipeline before pushing images?

Logging in authenticates the pipeline with the registry, allowing it to upload images securely and preventing unauthorized access.

intermediate

What environment variables are commonly used in CI pipelines for Docker registry authentication?

Variables like DOCKER_USERNAME and DOCKER_PASSWORD or tokens are used to store credentials securely.

intermediate

How can tagging Docker images in CI help with version control?

Tagging images with build numbers, commit hashes, or version numbers helps identify and deploy specific image versions easily.

What is the first step before pushing a Docker image in a CI pipeline?

A Logging in to the Docker registry

B Running docker push command

C Building the Docker image

D Tagging the Docker image

You must log in to the Docker registry first to authenticate before pushing images.

Which command tags a Docker image with a version before pushing?

A docker login

B docker push image:tag

C docker tag source-image target-image:tag

D docker build -t image:tag

The docker tag command assigns a tag to an existing image.

Why is it important to use environment variables for Docker credentials in CI?

A To speed up the build process

B To keep credentials secure and not hard-coded in scripts

C To avoid logging in to the registry

D To automatically build images

Environment variables protect sensitive data by not exposing credentials in code.

What happens if you try to push a Docker image without logging in first?

A Image will push successfully

B Docker will build the image automatically

C Image will be deleted

D Push will fail with an authentication error

Without login, the registry denies access and push fails.

Which of these is a common container registry where images can be pushed?

A Docker Hub

B GitHub

C Jenkins

D Travis CI

Docker Hub is a popular public container registry for storing Docker images.

Explain the steps to push a Docker image from a CI pipeline.

Think about what happens from building to uploading the image.

Why is tagging Docker images important in continuous integration workflows?

Consider how you keep track of different builds.

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