Image naming conventions (registry/image:tag) in Docker - Step-by-Step Execution

Process Flow - Image naming conventions (registry/image:tag)

Start with registry (optional)

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Add image name (required)

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Add tag (optional, default 'latest')

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Full image name formed

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Use image in docker commands

Docker image names can include an optional registry, a required image name, and an optional tag. The tag defaults to 'latest' if omitted.

Execution Sample

Docker

docker pull ubuntu:20.04

docker pull myregistry.com/myapp:1.2

docker pull nginx

These commands pull images using different naming conventions: with tag, with registry and tag, and with default tag.

Process Table

Step	Image Name Input	Registry Parsed	Image Parsed	Tag Parsed	Final Image Reference
1	ubuntu:20.04	docker.io (default)	ubuntu	20.04	docker.io/library/ubuntu:20.04
2	myregistry.com/myapp:1.2	myregistry.com	myapp	1.2	myregistry.com/myapp:1.2
3	nginx	docker.io (default)	nginx	latest (default)	docker.io/library/nginx:latest
4	exit	-	-	-	All images parsed and ready for use

💡 No more image names to parse; process ends.

Status Tracker

Variable	Start	After 1	After 2	After 3	Final
registry	none	docker.io	myregistry.com	docker.io	varies per input
image	none	ubuntu	myapp	nginx	varies per input
tag	none	20.04	1.2	latest	varies per input

Key Moments - 2 Insights

Why does 'nginx' get 'docker.io' as the registry and 'latest' as the tag?

When no registry is specified, Docker assumes the default 'docker.io'. When no tag is given, Docker uses 'latest' by default, as shown in execution_table row 3.

How does Docker know where the registry ends and the image name begins?

Docker treats the first part before the first slash as the registry if it contains a dot or colon (like 'myregistry.com'). Otherwise, it assumes the default registry. This is shown in execution_table rows 1 and 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the tag parsed for the image 'ubuntu:20.04'?

A 20.04

B latest

C ubuntu

D docker.io

💡 Hint

Check the 'Tag Parsed' column in row 1 of the execution_table.

At which step does Docker use the default registry 'docker.io'?

A Step 1 only

B Step 2 only

C Steps 1 and 3

D All steps

💡 Hint

Look at the 'Registry Parsed' column in the execution_table rows 1 and 3.

If the image name is 'myregistry.com/myapp' without a tag, what would the tag be?

A myapp

B latest

C none

D 1.0

💡 Hint

Tags default to 'latest' if omitted, as shown in the explanation and execution_table.

Concept Snapshot

Docker image names can include an optional registry, a required image name, and an optional tag.
Format: [registry/]<image_name>[:tag]
If registry is missing, 'docker.io' is assumed.
If tag is missing, 'latest' is assumed.
Examples: 'ubuntu:20.04', 'myregistry.com/myapp:1.2', 'nginx'
Use full image names in docker commands for clarity.

Full Transcript

Docker images are named using a pattern that can include a registry, an image name, and a tag. The registry is where the image is stored, like a website address. If you don't write a registry, Docker assumes the default one called 'docker.io'. The image name is required and tells Docker which image to use. The tag is optional and tells Docker which version of the image to use. If you don't write a tag, Docker uses 'latest' by default. For example, 'ubuntu:20.04' means the 'ubuntu' image with the '20.04' tag from the default registry. 'myregistry.com/myapp:1.2' means the 'myapp' image with tag '1.2' from 'myregistry.com'. 'nginx' means the 'nginx' image with the 'latest' tag from the default registry. Understanding this helps you pull and run the right images in Docker.

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