Docker Hub as image registry - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Docker login command output

What is the expected output after successfully logging into Docker Hub using the command docker login?

Docker

docker login

A Cannot connect to the Docker daemon

B Error: unauthorized: incorrect username or password

C Login Succeeded

D Usage: docker login [OPTIONS] SERVER

💡 Hint

Think about what message confirms a successful login.

When you run 'docker login' and provide correct credentials, Docker Hub responds with 'Login Succeeded'. Other messages indicate errors or incorrect usage.

🧠 Conceptual

intermediate

Purpose of Docker Hub repository tags

Why do we use tags in Docker Hub repositories?

A To identify different versions or variants of the same image

B To encrypt the Docker images for security

C To limit the number of downloads per image

D To automatically update the image on all servers

💡 Hint

Think about how you keep track of different versions of a file.

Tags help label images with versions or variants, making it easy to pull the exact image you want. They do not encrypt, limit downloads, or auto-update images.

🔀 Workflow

advanced

Correct sequence to push a Docker image to Docker Hub

Arrange the steps in the correct order to push a Docker image named myapp with tag v1 to Docker Hub.

A 3,4,1,2

B 4,3,1,2

C 4,1,3,2

D 3,1,4,2

💡 Hint

You must login before pushing, and tagging happens after building.

First, login to Docker Hub (4). Then build the image locally (3). Next, tag the image with your Docker Hub username (1). Finally, push the tagged image to Docker Hub (2).

❓ Troubleshoot

advanced

Error when pushing image: denied: requested access to the resource is denied

You get the error denied: requested access to the resource is denied when running docker push username/myapp:v1. What is the most likely cause?

A Your internet connection is down

B The Docker daemon is not running on your machine

C The image name contains invalid characters

D You are not logged in or logged in with a different Docker Hub account

💡 Hint

Think about permissions and authentication with Docker Hub.

This error usually means you are not authenticated properly or trying to push to a repository you don't have permission for. Other options cause different errors.

✅ Best Practice

expert

Recommended practice for managing sensitive data in Docker images pushed to Docker Hub

Which practice is best to avoid exposing sensitive data when pushing Docker images to Docker Hub?

A Use multi-stage builds to exclude secrets from the final image

B Include secrets in environment variables inside the Dockerfile

C Push images with all configuration files including passwords

D Store secrets in plain text files inside the image

💡 Hint

Think about how to keep secrets out of the final image.

Multi-stage builds allow you to copy only necessary artifacts into the final image, leaving secrets out. Including secrets in Dockerfiles or images risks exposure.

Practice

1. What is Docker Hub primarily used for in Docker workflows?

easy

A. A public registry to store and share Docker images

B. A tool to build Docker images locally

C. A command-line interface for managing containers

D. A service to monitor running Docker containers

Solution

1. Step 1: Understand Docker Hub's role

   Docker Hub is a cloud-based service where users can upload and download Docker images.

2. Step 2: Differentiate from other Docker tools

   Building images is done locally with Docker CLI, not Docker Hub. Docker Hub is for storing and sharing images.

3. Final Answer:

   A public registry to store and share Docker images -> Option A

4. Quick Check:

   Docker Hub = Image registry [OK]

Hint: Docker Hub = place to store/share images [OK]

Common Mistakes:

• Confusing Docker Hub with Docker CLI commands
• Thinking Docker Hub builds images
• Assuming Docker Hub manages running containers

2. Which command correctly logs you into Docker Hub from the terminal?

easy

A. docker login

B. docker push

C. docker pull

D. docker build

Solution

1. Step 1: Identify the login command

   The command docker login prompts for Docker Hub credentials to authenticate the user.

2. Step 2: Understand other commands

   docker push uploads images, docker pull downloads images, and docker build creates images locally.

3. Final Answer:

   docker login -> Option A

4. Quick Check:

   Login command = docker login [OK]

Hint: Use 'docker login' to authenticate with Docker Hub [OK]

Common Mistakes:

• Using 'docker push' to login
• Confusing 'docker pull' with login
• Trying to login with 'docker build'

3. What will happen if you run docker push myuser/myapp:latest without logging into Docker Hub first?

medium

A. Docker will create a new repository automatically

B. Docker will prompt for login credentials

C. The image will be pushed successfully

D. The command will fail with an authentication error

Solution

1. Step 1: Understand authentication requirement for push

   Pushing images to Docker Hub requires authentication; without login, Docker cannot verify permissions.

2. Step 2: Result of pushing without login

   Docker will reject the push and show an authentication error instead of prompting or succeeding.

3. Final Answer:

   The command will fail with an authentication error -> Option D

4. Quick Check:

   Push without login = auth error [OK]

Hint: Push needs login; no login causes auth error [OK]

Common Mistakes:

• Expecting Docker to prompt for login automatically
• Assuming push works without authentication
• Thinking Docker creates repos automatically on push

4. You tried to push an image with docker push myuser/myapp but got an error: denied: requested access to the resource is denied. What is the most likely cause?

medium

A. You forgot to tag the image with your Docker Hub username

B. Docker daemon is not running

C. You are not logged into Docker Hub or lack permission

D. The image name contains invalid characters

Solution

1. Step 1: Analyze the error message

   The error means Docker Hub rejected access, usually due to missing login or permission issues.

2. Step 2: Check common causes

   Not being logged in or lacking rights to push to that repository causes this error, even if tagging is correct.

3. Final Answer:

   You are not logged into Docker Hub or lack permission -> Option C

4. Quick Check:

   Access denied = login or permission issue [OK]

Hint: Access denied usually means login or permission problem [OK]

Common Mistakes:

• Assuming tagging is always the cause
• Ignoring login status
• Thinking Docker daemon stopped causes this error

5. You want to share a custom Docker image named myapp with your team using Docker Hub. Which sequence of commands correctly tags and pushes the image to your Docker Hub repository myuser/myapp?

hard

A. docker login && docker build myapp && docker push myuser/myapp

B. docker tag myapp myuser/myapp:latest && docker push myuser/myapp:latest

C. docker push myapp && docker tag myuser/myapp:latest

D. docker pull myapp && docker tag myuser/myapp && docker push

Solution

1. Step 1: Tag the local image with your Docker Hub repo name

   Use docker tag myapp myuser/myapp:latest to prepare the image for pushing.

2. Step 2: Push the tagged image to Docker Hub

   Run docker push myuser/myapp:latest to upload the image to your Docker Hub repository.

3. Final Answer:

   docker tag myapp myuser/myapp:latest && docker push myuser/myapp:latest -> Option B

4. Quick Check:

   Tag then push = correct sequence [OK]

Hint: Tag image with repo name before pushing [OK]

Common Mistakes:

• Pushing without tagging the image
• Pushing before tagging or building
• Confusing pull with push commands