Pushing images from CI in Docker - Step-by-Step Execution

Process Flow - Pushing images from CI

Start CI Pipeline

↓

Build Docker Image

↓

Login to Docker Registry

↓

Push Image to Registry

↓

Confirm Push Success

↓

End CI Pipeline

This flow shows how a CI pipeline builds a Docker image, logs into a registry, pushes the image, and confirms success.

Execution Sample

Docker

docker build -t myapp:latest .
docker login -u $DOCKER_USER -p $DOCKER_PASS

docker push myapp:latest

Builds a Docker image, logs into the registry using credentials, then pushes the image.

Process Table

Step	Action	Command	Result	Notes
1	Build Image	docker build -t myapp:latest .	Image 'myapp:latest' built successfully	Dockerfile used in current directory
2	Login to Registry	docker login -u $DOCKER_USER -p $DOCKER_PASS	Login successful	Credentials from CI environment variables
3	Push Image	docker push myapp:latest	Image pushed to registry	Image available in remote registry
4	Verify Push	docker pull myapp:latest	Image pulled successfully	Confirms image is stored remotely
5	End	-	CI pipeline completes	All steps succeeded

💡 Pipeline ends after successful image push and verification

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Image Tag	none	myapp:latest	myapp:latest	myapp:latest	myapp:latest	myapp:latest
Login Status	not logged in	not logged in	logged in	logged in	logged in	logged in
Push Status	not pushed	not pushed	not pushed	pushed	pushed	pushed
Pull Status	not pulled	not pulled	not pulled	not pulled	pulled	pulled

Key Moments - 3 Insights

Why do we need to login before pushing the image?

Because the registry requires authentication to allow image uploads. Step 2 in the execution_table shows login success before pushing.

What happens if the image build fails?

The pipeline stops early. Step 1 must succeed to proceed. If build fails, no login or push happens.

Why verify the image by pulling after push?

To confirm the image is correctly stored in the remote registry. Step 4 shows pulling the image to verify.

Visual Quiz - 3 Questions

Look at the execution table, what is the login status after Step 2?

A logged in

B not logged in

C login failed

D unknown

💡 Hint

Check the 'Login Status' variable after Step 2 in variable_tracker

At which step does the image get pushed to the registry?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Push Image' action in the execution_table

If the login fails, what will happen to the push step?

A Push will succeed anyway

B Push will be skipped or fail

C Push will retry automatically

D Push will push to a different registry

💡 Hint

Refer to key_moments about login necessity before push

Concept Snapshot

Pushing images from CI:
1. Build image with 'docker build -t name:tag .'
2. Login to registry using 'docker login' with credentials
3. Push image using 'docker push name:tag'
4. Verify push by pulling image
Ensure credentials are set in CI environment variables.

Full Transcript

In a CI pipeline, first the Docker image is built using the docker build command. Then the pipeline logs into the Docker registry using credentials stored securely in environment variables. After successful login, the image is pushed to the remote registry. Finally, the pipeline verifies the push by pulling the image back. If any step fails, the pipeline stops to prevent errors. This process automates image delivery from code changes to deployment-ready images stored remotely.

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