Image naming conventions (registry/image:tag) in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Docker image naming format

Which part of the Docker image name registry.example.com/myapp/backend:1.2.3 specifies the version of the image?

A 1.2.3

B myapp/backend

C myapp

D registry.example.com

💡 Hint

The version is usually indicated after a colon at the end.

The part after the colon (:1.2.3) is the tag, which commonly represents the version of the image.

💻 Command Output

intermediate

Output of docker pull with full image name

What will be the output status when running docker pull registry.example.com/myapp/frontend:latest if the image exists in the registry?

A Error: image not found

B

latest: Pulling from myapp/frontend
Digest: sha256:abc123...
Status: Downloaded newer image for registry.example.com/myapp/frontend:latest

C Status: Image is up to date for registry.example.com/myapp/frontend:latest

D Permission denied

💡 Hint

Pulling an image shows download progress and digest.

When pulling an image that exists, Docker shows the pulling progress and digest, then confirms the image is downloaded.

❓ Configuration

advanced

Correct image name for private registry

Which of the following is the correct Docker image name to push an image to a private registry at myregistry.local:5000 with repository project/api and tag v5?

A project/api:v5

B myregistry.local/project/api:v5

C myregistry.local:5000/project/api:v5

D myregistry.local:project/api:v5

💡 Hint

Private registries often include port numbers after a colon.

The correct format includes the registry hostname with port, followed by repository and tag: myregistry.local:5000/project/api:v5.

❓ Troubleshoot

advanced

Error caused by invalid image tag format

What error will Docker show if you try to tag an image with an invalid tag containing uppercase letters, like myapp/backend:VersionOne?

A Error response from daemon: invalid reference format

B Tag created successfully

C Error: tag contains invalid characters

D Warning: tag converted to lowercase

💡 Hint

Docker tags must be lowercase and follow specific rules.

Docker enforces tag format rules and will reject tags with uppercase letters, showing 'invalid reference format' error.

✅ Best Practice

expert

Choosing the best tag for continuous deployment

In a continuous deployment pipeline, which tag naming strategy is best to ensure traceability and avoid overwriting images unintentionally?

A Use the 'latest' tag for every build

B Use random strings as tags for each build

C Use timestamps as tags like '20240615-1530'

D Use semantic versioning tags like 'v1.2.3' for each build

💡 Hint

Tags should help identify the exact version deployed.

Semantic versioning tags provide clear version info and help track deployments. 'latest' can be overwritten and cause confusion.

Practice

1. Which part of the Docker image name docker.io/library/nginx:latest specifies the version of the image?

easy

A. latest

B. docker.io

C. nginx

D. library

Solution

1. Step 1: Identify the tag in the image name

   The tag is the part after the colon (:), which specifies the version or variant of the image.

2. Step 2: Locate the tag in the example

   In docker.io/library/nginx:latest, the tag is latest.

3. Final Answer:

   latest -> Option A

4. Quick Check:

   Tag = latest [OK]

Hint: Tag is always after colon ':' in image name [OK]

Common Mistakes:

• Confusing registry with tag
• Thinking image name is the tag
• Ignoring the colon separator

2. Which of the following is the correct syntax for a Docker image name with a registry, image, and tag?

easy

A. tag/image:registry

B. registry/image:tag

C. image:tag/registry

D. image/registry:tag

Solution

1. Step 1: Recall the Docker image naming format

   The correct format is registry/image:tag, where registry is optional but comes first, then image name, then tag after colon.

2. Step 2: Match the options to the format

   Only registry/image:tag matches the correct order and separators.

3. Final Answer:

   registry/image:tag -> Option B

4. Quick Check:

   Format = registry/image:tag [OK]

Hint: Registry first, then image, then tag after ':' [OK]

Common Mistakes:

• Placing tag before image
• Swapping registry and image order
• Using wrong separators like '/' for tag

3. What is the full image name including registry and tag for the official Redis image version 7.0 from Docker Hub?

medium

A. docker.io/redis7:0

B. redis:7.0

C. redis/docker.io:7.0

D. docker.io/redis:7.0

Solution

1. Step 1: Understand Docker Hub default registry

   Docker Hub's registry is docker.io. The official images are under this registry.

2. Step 2: Compose full image name

   The full name includes registry, image name, and tag: docker.io/redis:7.0.

3. Final Answer:

   docker.io/redis:7.0 -> Option D

4. Quick Check:

   Full name = docker.io/redis:7.0 [OK]

Hint: Full name = registry/image:tag, Docker Hub registry is docker.io [OK]

Common Mistakes:

• Putting registry after image
• Missing registry for full name
• Incorrect tag format

4. You try to pull the image myregistry.com/myapp but get an error. Which of these is a likely cause?

medium

A. Missing tag defaults to 'latest', but image may not have it

B. Registry name cannot contain dots '.'

C. Image name must always include a tag

D. Registry must be 'docker.io' only

Solution

1. Step 1: Understand default tag behavior

   If no tag is given, Docker uses latest by default. If the image does not have a 'latest' tag, pull fails.

2. Step 2: Check other options for errors

   Registry names can contain dots, tags are optional, and registries can be other than docker.io.

3. Final Answer:

   Missing tag defaults to 'latest', but image may not have it -> Option A

4. Quick Check:

   Default tag 'latest' may be missing [OK]

Hint: No tag means 'latest' assumed, may cause pull failure [OK]

Common Mistakes:

• Thinking registry can't have dots
• Believing tag is mandatory
• Assuming only docker.io registry works

5. You want to tag a local image myapp to push it to a private registry at registry.example.com with version v2. Which command correctly tags the image?

hard

A. docker tag myapp:v2 registry.example.com/myapp

B. docker tag registry.example.com/myapp:v2 myapp

C. docker tag myapp registry.example.com/myapp:v2

D. docker tag myapp registry.example.com:v2/myapp

Solution

1. Step 1: Understand docker tag syntax

   The syntax is docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG].

2. Step 2: Apply to given names

   Source is myapp (local), target is registry.example.com/myapp:v2 to specify registry and tag.

3. Final Answer:

   docker tag myapp registry.example.com/myapp:v2 -> Option C

4. Quick Check:

   Tag command: source then target with registry and tag [OK]

Hint: Tag command: source image then target with registry:tag [OK]

Common Mistakes:

• Swapping source and target images
• Placing tag in wrong position
• Using invalid registry/tag format