Image tags and versioning in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Understanding Docker image tag output

What is the output of the command docker images --format "{{.Repository}}:{{.Tag}}" | grep '^myapp:' if the local images are:

• myapp:latest
• myapp:1.0
• myapp:1.1
• otherapp:2.0

Docker

docker images --format "{{.Repository}}:{{.Tag}}" | grep '^myapp:'

A No output

B

myapp:latest
myapp:1.0
myapp:1.1
otherapp:2.0

C otherapp:2.0

D

myapp:latest
myapp:1.0
myapp:1.1

💡 Hint

The grep command filters lines starting with 'myapp:'

The command lists all images with their repository and tag, then grep filters only those starting with 'myapp:'. So only the three 'myapp' tags appear.

🧠 Conceptual

intermediate

Docker image tag best practice

Which of the following is the best reason to avoid using the latest tag in production deployments?

A The latest tag is slower to download than versioned tags

B The latest tag always points to the newest image, which can cause unexpected updates breaking the app

C The latest tag cannot be pushed to Docker Hub

D The latest tag uses more disk space on the host

💡 Hint

Think about stability and predictability in production

Using 'latest' means you might pull a different image version than expected, causing unpredictable behavior. Versioned tags ensure you deploy a known image.

🔀 Workflow

advanced

Tagging and pushing multiple image versions

You built a Docker image locally with ID abc123. You want to tag it as myapp:1.0 and myapp:stable and push both tags to Docker Hub. Which sequence of commands correctly does this?

A

docker tag myapp:1.0 abc123
docker push myapp:1.0
 docker push myapp:stable

B

docker push abc123 myapp:1.0
 docker push abc123 myapp:stable

C

docker tag abc123 myapp:1.0
docker tag abc123 myapp:stable
docker push myapp:1.0
docker push myapp:stable

D

docker build -t myapp:1.0 .
docker push myapp:stable

💡 Hint

Remember to tag the image ID before pushing tags

You must tag the image ID with each tag, then push each tag separately. Option C does this correctly.

❓ Troubleshoot

advanced

Troubleshooting image pull failure with tags

You run docker pull myapp:2.0 but get the error manifest for myapp:2.0 not found. What is the most likely cause?

A The tag '2.0' does not exist in the remote repository

B The image name 'myapp' is invalid

C You have no internet connection

D Docker daemon is not running

💡 Hint

Check if the tag exists remotely

This error means Docker cannot find the specified tag in the remote repo. The tag might not have been pushed or was deleted.

✅ Best Practice

expert

Versioning strategy for Docker images in CI/CD

In a CI/CD pipeline, which tagging strategy best supports traceability and rollback of Docker images?

A Tag images with the Git commit SHA and push with that tag

B Always tag images as 'latest' to simplify deployment

C Use timestamps as tags without linking to source code

D Tag images only with semantic version numbers like 1.0, 1.1

💡 Hint

Think about linking image versions to exact code changes

Tagging with Git commit SHA links the image to the exact source code version, enabling precise traceability and rollback.

Practice

1. What is the default tag used by Docker if no tag is specified when pulling an image?

easy

A. stable

B. 1.0

C. latest

D. default

Solution

1. Step 1: Understand Docker image tagging

   Docker images can have tags to specify versions or variants.

2. Step 2: Identify the default tag behavior

   If no tag is specified, Docker uses the tag named "latest" by default.

3. Final Answer:

   latest -> Option C

4. Quick Check:

   Default tag = latest [OK]

Hint: No tag means Docker uses 'latest' automatically [OK]

Common Mistakes:

• Assuming 'stable' is default tag
• Thinking '1.0' is default version
• Believing 'default' is a valid tag

2. Which of the following is the correct syntax to pull a Docker image with a specific tag?

easy

A. docker pull ubuntu:20.04

B. docker pull ubuntu-20.04

C. docker pull ubuntu/20.04

D. docker pull ubuntu#20.04

Solution

1. Step 1: Recall Docker image tag syntax

   The correct syntax to specify a tag is image_name:tag.

2. Step 2: Match syntax with options

   Only docker pull ubuntu:20.04 uses a colon to separate image name and tag correctly.

3. Final Answer:

   docker pull ubuntu:20.04 -> Option A

4. Quick Check:

   Use colon ':' to specify tag [OK]

Hint: Use ':' between image name and tag [OK]

Common Mistakes:

• Using dash '-' instead of colon ':'
• Using slash '/' instead of colon
• Using hash '#' instead of colon

3. Given the command docker pull nginx:1.21, what will be the result?

medium

A. Docker throws an error due to invalid tag

B. Docker pulls the latest nginx image ignoring 1.21

C. Docker pulls all nginx images

D. Docker pulls the nginx image tagged 1.21

Solution

1. Step 1: Analyze the pull command with tag

   The command specifies the image name 'nginx' and tag '1.21'.

2. Step 2: Understand Docker pull behavior

   Docker pulls the exact image version matching the tag '1.21'.

3. Final Answer:

   Docker pulls the nginx image tagged 1.21 -> Option D

4. Quick Check:

   Tag specifies exact image version [OK]

Hint: Tag pulls specific image version, not latest [OK]

Common Mistakes:

• Assuming tag is ignored and latest is pulled
• Expecting error for valid tag
• Thinking all images are pulled

4. You run docker pull alpine and get an error that the image is not found. What is the most likely cause?

medium

A. You forgot to specify the tag, and 'latest' tag is missing

B. The image 'alpine' does not exist on Docker Hub

C. Docker requires a tag to pull any image

D. Your internet connection is down

Solution

1. Step 1: Understand default tag behavior

   When no tag is specified, Docker tries to pull the 'latest' tag.

2. Step 2: Identify cause of error

   If the 'latest' tag does not exist for 'alpine', Docker cannot find the image.

3. Final Answer:

   You forgot to specify the tag, and 'latest' tag is missing -> Option A

4. Quick Check:

   Missing 'latest' tag causes pull failure [OK]

Hint: Always specify tag if 'latest' is missing [OK]

Common Mistakes:

• Assuming image does not exist at all
• Thinking tag is always required
• Blaming internet without checking tags

5. You want to deploy a Docker image but ensure you always use version 2.5.3, never the latest. Which command correctly pulls this specific version and avoids accidental updates?

hard

A. docker pull myapp:stable

B. docker pull myapp:2.5.3

C. docker pull myapp:latest

D. docker pull myapp

Solution

1. Step 1: Understand version control with tags

   Using a specific tag like '2.5.3' ensures you get that exact version.

2. Step 2: Compare options for safety

   docker pull myapp:2.5.3 pulls the exact version. Options B and C pull 'latest' which can change. docker pull myapp:stable depends on tag existence.

3. Final Answer:

   docker pull myapp:2.5.3 -> Option B

4. Quick Check:

   Explicit version tag prevents accidental updates [OK]

Hint: Always specify exact version tag to avoid surprises [OK]

Common Mistakes:

• Using 'latest' tag and getting unexpected updates
• Assuming 'stable' tag always exists
• Not specifying any tag and risking version changes