Document multi-platform Docker builds with buildx

[adnaan]

Context

This follow-up task was identified during the review of PR #132.

Source PR: #132
PR Title: feat: add Docker support with multi-stage build
Suggested by: @claude[bot]

Task Description

Document how to build multi-platform images using Docker buildx for supporting both AMD64 and ARM64 architectures.

Example:

docker buildx build --platform linux/amd64,linux/arm64 -t tinkerdown:latest .

Acceptance Criteria

• Documentation includes buildx commands
• Explains platform options

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This issue was automatically created from PR review comments.