Pushing images to registry in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Docker push command?

You run the command docker push myrepo/myimage:latest but you are not logged in. What output will you see?

Docker

docker push myrepo/myimage:latest

A Error response from daemon: unauthorized: authentication required

B Pushed image successfully to myrepo/myimage:latest

C Error: No such image: myrepo/myimage:latest

D Warning: tag latest not found in repository myrepo/myimage

💡 Hint

Think about what happens if you try to push without logging in first.

Docker requires authentication to push images to a registry. Without logging in, the push command fails with an unauthorized error.

🧠 Conceptual

intermediate

Which step is required before pushing a Docker image to a private registry?

You want to push your Docker image to a private registry. What must you do first?

A Run <code>docker pull</code> to download the image

B Run <code>docker login</code> with the registry credentials

C Run <code>docker build</code> with the <code>--push</code> flag

D Run <code>docker logout</code> to clear old credentials

💡 Hint

Think about how Docker authenticates with registries.

You must authenticate with the registry using docker login before pushing images to it.

❓ Troubleshoot

advanced

Why does this Docker push fail with 'denied: requested access to the resource is denied'?

You tagged your image as myrepo/myimage:latest and ran docker push myrepo/myimage:latest. The push fails with:
denied: requested access to the resource is denied
What is the most likely cause?

A You do not have permission to push to the repository 'myrepo/myimage'

B You forgot to run 'docker pull' before pushing

C Docker daemon is not running

D The image tag 'latest' does not exist locally

💡 Hint

Consider repository permissions and ownership.

This error usually means your user account lacks permission to push to that repository on the registry.

🔀 Workflow

advanced

Order the steps to push a Docker image to Docker Hub

Put these steps in the correct order to push a Docker image to Docker Hub:

A 4,3,1,2

B 3,1,4,2

C 3,4,1,2

D 1,3,4,2

💡 Hint

You must authenticate before pushing, but after building the image.

First build the image, then log in, then tag it properly, then push.

✅ Best Practice

expert

Which practice helps avoid pushing large unused layers to a registry?

You want to optimize your Docker image pushes to reduce size and speed. Which practice is best?

A Use 'docker push --compress=false' to speed up pushing

B Always push images with the 'latest' tag

C Run 'docker system prune' before pushing

D Use multi-stage builds to keep only necessary layers

💡 Hint

Think about how to reduce image size before pushing.

Multi-stage builds allow you to discard build-time layers, keeping images smaller and push faster.

Practice

1.

What is the main purpose of the docker push command?

easy

A. Remove a Docker image from local storage

B. Download an image from a remote registry

C. Run a container from an image

D. Upload a local Docker image to a remote registry

Solution

1. Step 1: Understand the command purpose

   The docker push command is used to send a local image to a remote registry so others can access it.

2. Step 2: Compare options with command function

   Downloading images uses docker pull, running containers uses docker run, and removing images uses docker rmi. Only uploading matches docker push.

3. Final Answer:

   Upload a local Docker image to a remote registry -> Option D

4. Quick Check:

   Push means upload image [OK]

Hint: Push means upload your image to share it [OK]

Common Mistakes:

• Confusing push with pull (download)
• Thinking push runs containers
• Assuming push deletes images

2.

Which of the following is the correct syntax to push a Docker image named myapp with tag v1 to Docker Hub?

docker push ________

easy

A. myapp:v1

B. myapp

C. myapp:v1 latest

D. push myapp:v1

Solution

1. Step 1: Recall push command syntax

   The correct syntax is docker push [repository]:[tag]. The tag is required if not using default 'latest'.

2. Step 2: Analyze options

   myapp:v1 uses myapp:v1 which is correct. myapp misses the tag. myapp:v1 latest has extra word 'latest' which is invalid. push myapp:v1 repeats 'push' which is wrong.

3. Final Answer:

   myapp:v1 -> Option A

4. Quick Check:

   Push needs image name and tag [OK]

Hint: Use image name with tag like repo:tag [OK]

Common Mistakes:

• Omitting the tag when required
• Adding extra words after image name
• Repeating the push command inside

3.

Given the commands below, what will be the output of the docker push command?

docker tag myapp:latest myrepo/myapp:v2
docker push myrepo/myapp:v2

medium

A. Image pushed successfully to myrepo/myapp:v2

B. Error: image not found locally

C. Authentication required before push

D. Image pushed but with tag 'latest'

Solution

1. Step 1: Understand tagging and pushing

   The image myapp:latest is tagged as myrepo/myapp:v2, preparing it for push to myrepo registry.

2. Step 2: Predict push output

   Since the image is tagged correctly and assuming user is logged in, the push will succeed with a success message.

3. Final Answer:

   Image pushed successfully to myrepo/myapp:v2 -> Option A

4. Quick Check:

   Tag then push means success [OK]

Hint: Tag image with repo before push [OK]

Common Mistakes:

• Forgetting to tag image before push
• Not logging in to registry
• Assuming default tag is always 'latest'

4.

What is wrong with the following commands if the push fails?

docker push myapp:v1

Assuming myapp:v1 is a local image and the user wants to push to Docker Hub.

medium

A. The push command syntax is incorrect

B. Image is not tagged with the Docker Hub repository name

C. Docker daemon is not running

D. The image tag 'v1' is invalid

Solution

1. Step 1: Check image tagging for remote push

   To push to Docker Hub, the image must be tagged with the repository name like username/myapp:v1.

2. Step 2: Identify the cause of failure

   Since the image is only myapp:v1 without repo prefix, Docker Hub rejects the push.

3. Final Answer:

   Image is not tagged with the Docker Hub repository name -> Option B

4. Quick Check:

   Push needs repo prefix [OK]

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

Common Mistakes:

• Trying to push without repo prefix
• Assuming push syntax is wrong
• Ignoring Docker daemon status

5.

You want to push a local image webapp:latest to a private registry at registry.example.com. Which sequence of commands is correct?

hard

A. docker login registry.example.comdocker push webapp:latest

B. docker push webapp:latestdocker login registry.example.com

C. docker login registry.example.comdocker tag webapp:latest registry.example.com/webapp:latestdocker push registry.example.com/webapp:latest

D. docker tag webapp:latest registry.example.com/webappdocker push webapp:latest

Solution

1. Step 1: Login to the private registry

   You must log in first to authenticate with registry.example.com.

2. Step 2: Tag the image with the registry URL

   Tag webapp:latest as registry.example.com/webapp:latest so Docker knows where to push.

3. Step 3: Push the tagged image to the registry

   Push the correctly tagged image to upload it to the private registry.

4. Final Answer:

   docker login registry.example.com docker tag webapp:latest registry.example.com/webapp:latest docker push registry.example.com/webapp:latest -> Option C

5. Quick Check:

   Login, tag with registry, then push [OK]

Hint: Login, tag with registry URL, then push [OK]

Common Mistakes:

• Pushing before login
• Not tagging with registry URL
• Pushing wrong image name