Image naming conventions (registry/image:tag) in Docker - Mini Project: Build & Apply

Image naming conventions (registry/image:tag)

📖 Scenario: You are working with Docker images and need to understand how to name them properly. Docker images use a naming format that helps you identify where the image is stored, what the image is, and which version it is.

🎯 Goal: Learn how to create and use Docker image names with the correct format: registry/image:tag. You will build a simple example of naming an image with a registry, image name, and tag.

📋 What You'll Learn

Create a variable with a Docker image name using the format registry/image:tag

Add a variable for the default registry

Extract the image name and tag from the full image name

Print the extracted image name and tag

💡 Why This Matters

🌍 Real World

Docker images are used to package applications and their environments. Knowing how to name and tag images helps you manage versions and sources clearly.

💼 Career

Understanding image naming conventions is essential for DevOps roles that involve container management, deployment pipelines, and working with container registries.

1

Create a Docker image name variable

Create a variable called full_image_name and set it to the string docker.io/library/ubuntu:22.04.

Docker

# Create a variable called full_image_name with the value 'docker.io/library/ubuntu:22.04'
# Your code here

Hint

Remember to use quotes around the string value.

2

Add a default registry variable

Create a variable called default_registry and set it to the string docker.io.

Docker

full_image_name = "docker.io/library/ubuntu:22.04"
# Create a variable called default_registry with the value 'docker.io'
# Your code here

Hint

This variable will help identify the registry part of the image name.

3

Extract image name and tag from full image name

Use the split method on full_image_name to create two variables: image_name for the part after the registry and before the colon, and tag for the part after the colon. For example, image_name should be library/ubuntu and tag should be 22.04.

Docker

full_image_name = "docker.io/library/ubuntu:22.04"
default_registry = "docker.io"
# Extract image_name and tag from full_image_name
# Your code here

Hint

First split by '/' to remove the registry, then split by ':' to separate the tag.

4

Print the extracted image name and tag

Write two print statements to display the values of image_name and tag.

Docker

full_image_name = "docker.io/library/ubuntu:22.04"
default_registry = "docker.io"
image_with_tag = full_image_name.split("/", 1)[1]
image_name, tag = image_with_tag.split(":")
# Print image_name and tag
# Your code here

Hint

Use print(image_name) and print(tag) to show the results.

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