fix: lowercase registry, too

[salim-b]

Description

image and tags are automatically lowercased to be OCI spec compliant, but registry is not. Because according to the spec, the repository namespace must also be lowercase (and the host is case-insensitive per DNS spec), registry should be lowercased, too.

Related Issue(s)

Checklist

• This PR includes a documentation change
• This PR does not need a documentation change

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• This PR includes test changes
• This PR's changes are already tested

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• This change is not user-facing
• This change is a patch change
• This change is a minor change
• This change is a major (breaking) change

Changes made

Lowercased registry in src/index.ts and updated the check-lowercase.yaml test workflow as well as the README.md.