Add note about ghcr.io for HOMEBREW_DOCKER_REGISTRY_TOKEN

[azatoth]

It's not clear that when using HOMEBREW_DOCKER_REGISTRY_TOKEN directly against ghcr.io that you should use your PAT base64-encoded as the bearer token.

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same change?
• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes? Here's an example.
• [x] Have you successfully run brew style with your changes locally?
• [x] Have you successfully run brew typecheck with your changes locally?
• [x] Have you successfully run brew tests with your changes locally?

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[Bo98]

(Prior discussion for context: https://github.com/orgs/Homebrew/discussions/5171)

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[azatoth]

Would like to see comments addressed or further discussion, thanks!

I might have misunderstood you, but I thought I had addressed your comments, and I don't know what more I can say to further the issue.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.