Pulling from private registries in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a private Docker registry?

A private Docker registry is a secure place where you can store and share Docker images only with authorized users, unlike public registries which are open to everyone.

beginner

How do you authenticate Docker to pull images from a private registry?

You use the command docker login <registry_url> and enter your username and password to authenticate Docker with the private registry.

beginner

What command pulls an image from a private Docker registry after login?

Use docker pull <registry_url>/<image_name>:<tag> to pull the image from the private registry.

intermediate

Why might you get a 'denied: requested access to the resource is denied' error when pulling from a private registry?

This error usually means you are not logged in or do not have permission to access the image in the private registry.

beginner

What is the purpose of the docker logout command?

It removes your saved login credentials for a registry, so Docker will no longer authenticate automatically with that registry.

Which command do you use to authenticate Docker with a private registry?

A docker login <registry_url>

B docker pull <image_name>

C docker build <Dockerfile>

D docker logout

You must use 'docker login ' to provide your credentials before pulling images from a private registry.

What happens if you try to pull an image from a private registry without logging in?

A Docker shows an access denied error.

B Docker pulls the image successfully.

C Docker deletes the image.

D Docker automatically logs in.

Without login, Docker cannot authenticate and will deny access to private images.

How do you specify the private registry when pulling an image?

A docker pull <image_name>

B docker push <image_name>

C docker pull <registry_url>/<image_name>

D docker build <image_name>

You must include the registry URL before the image name to pull from a private registry.

What is the effect of running 'docker logout <registry_url>'?

A It updates Docker to the latest version.

B It logs you out from the Docker Hub only.

C It deletes all images from your local machine.

D It removes saved credentials for the specified registry.

'docker logout ' removes your saved login credentials for that registry.

Which of these is NOT a reason to use a private Docker registry?

A To share images only with authorized users.

B To publicly share images with everyone.

C To keep images secure and private.

D To control access to your Docker images.

Public sharing is the purpose of public registries, not private ones.

Explain the steps to pull a Docker image from a private registry.

Think about authentication first, then pulling the image.

Describe common errors when pulling from private registries and how to fix them.

Focus on authentication and command syntax.

Practice

1. What is the first step to pull an image from a private Docker registry?

easy

A. Run docker login with the registry URL to authenticate.

B. Directly run docker pull without authentication.

C. Create a Dockerfile to build the image locally.

D. Restart the Docker daemon.

Solution

1. Step 1: Understand private registry access

   Private registries require authentication before pulling images to ensure security.

2. Step 2: Use the correct command for authentication

   The docker login command with the registry URL authenticates the user.

3. Final Answer:

   Run docker login with the registry URL to authenticate. -> Option A

4. Quick Check:

   Authentication = docker login [OK]

Hint: Always login first to private registries before pulling images [OK]

Common Mistakes:

• Trying to pull without login causes permission errors
• Confusing image build with pulling
• Restarting Docker daemon is unrelated

2. Which of the following is the correct syntax to login to a private Docker registry at myregistry.example.com?

easy

A. docker login myregistry.example.com

B. docker pull myregistry.example.com

C. docker push myregistry.example.com

D. docker build myregistry.example.com

Solution

1. Step 1: Identify the login command

   The docker login command is used to authenticate to a registry.

2. Step 2: Use the registry URL as argument

   Adding the registry URL after docker login targets that specific private registry.

3. Final Answer:

   docker login myregistry.example.com -> Option A

4. Quick Check:

   Login syntax = docker login [OK]

Hint: Use 'docker login' plus registry URL to authenticate [OK]

Common Mistakes:

• Using pull or push commands instead of login
• Omitting the registry URL when needed
• Confusing build command with login

3. What will be the output if you run the following commands in order?

docker login myregistry.example.com
docker pull myregistry.example.com/myapp:latest

medium

A. Error: unauthorized: authentication required

B. Image 'myapp:latest' downloaded successfully from myregistry.example.com

C. Error: repository not found

D. Docker daemon restart required

Solution

1. Step 1: Authenticate with the private registry

   The docker login command authenticates the user successfully.

2. Step 2: Pull the image after authentication

   Since the user is logged in, docker pull downloads the private image without errors.

3. Final Answer:

   Image 'myapp:latest' downloaded successfully from myregistry.example.com -> Option B

4. Quick Check:

   Login then pull = success [OK]

Hint: Login first, then pull to avoid unauthorized errors [OK]

Common Mistakes:

• Pulling without login causes unauthorized errors
• Assuming image is public without checking
• Expecting daemon restart after login

4. You tried to pull an image from a private registry but got this error:
Error response from daemon: unauthorized: authentication required
What is the most likely cause and fix?

medium

A. The image name is misspelled; correct the image name.

B. Your internet connection is down; check network.

C. Docker daemon is not running; restart Docker service.

D. You forgot to run docker login before pulling; run login first.

Solution

1. Step 1: Analyze the error message

   The error says authentication is required, meaning no valid login was done.

2. Step 2: Apply the fix by authenticating

   Running docker login with the registry URL will authenticate and fix the error.

3. Final Answer:

   You forgot to run docker login before pulling; run login first. -> Option D

4. Quick Check:

   Unauthorized error = missing login [OK]

Hint: Unauthorized error means login missing; run docker login [OK]

Common Mistakes:

• Assuming image name typo causes unauthorized error
• Restarting Docker daemon unnecessarily
• Blaming network without checking login

5. You want to automate pulling a private image from registry.example.com in a script. Which sequence of commands correctly handles authentication and pulling?

hard

A. docker build registry.example.com/myimage:latest && docker login registry.example.com

B. docker pull registry.example.com/myimage:latest && docker login registry.example.com

C. docker login registry.example.com && docker pull registry.example.com/myimage:latest

D. docker logout registry.example.com && docker pull registry.example.com/myimage:latest

Solution

1. Step 1: Authenticate before pulling

   Use docker login first to authenticate to the private registry.

2. Step 2: Pull the image after successful login

   Run docker pull to download the private image after authentication.

3. Final Answer:

   docker login registry.example.com && docker pull registry.example.com/myimage:latest -> Option C

4. Quick Check:

   Login then pull = correct sequence [OK]

Hint: Login first, then pull in scripts using && [OK]

Common Mistakes:

• Pulling before login causes failure
• Using build instead of pull for images
• Logging out before pulling removes credentials