Add reusable guide for SSH key usage during Docker builds

[dewan-ahmed]

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Description

• Please describe your changes: This PR introduces a reusable documentation block for using SSH keys during Docker builds, and includes it across multiple "Build and Push to [X] Registry" pages.
• Jira/GitHub Issue numbers (if any): CI-17612
• Preview links/images (Internal contributors only):

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[SushrutHarness]

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