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.