Add copilot-instructions.md to guide infrastructure maintenance

[Copilot]

Plan for Adding copilot-instructions.md

• [x] Create .github/copilot-instructions.md file with repository-specific instructions
  • [x] Include information about the repository structure
  • [x] Reference the developer guide (eng/docker-tools/DEV-GUIDE.md) to ensure it stays updated
  • [x] Add build and test instructions
  • [x] Include information about the shared infrastructure pattern
• [x] Review and verify the copilot-instructions.md file
  • [x] Fixed code block syntax for PowerShell examples
  • [x] Simplified build and test commands with clearer examples
• [x] Add code review guidelines section
  • [x] Document when to update eng/docker-tools/DEV-GUIDE.md
  • [x] Document when to update README.md#building-locally
  • [x] Document when to update src/README.md
  • [x] Include general documentation update questions
• [x] Address review feedback
  • [x] Remove hardcoded SDK version number to avoid stale documentation
  • [x] Remove all code formatting and style preferences (enforced by .editorconfig and dotnet format)
• [x] Run final code review
• [x] Run security scan (no code changes to analyze)

Key Design Decision

Code formatting and style preferences are intentionally excluded from this document. All formatting rules are enforced by .editorconfig and fixable via dotnet format, preventing duplication and maintenance burden.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add copilot-instructions.md file for the repo</issue_title> <issue_description>This was mentioned in https://github.com/dotnet/docker-tools/pull/1889#issuecomment-3662475866, specifically to help ensure the developer guide is kept up to date.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes dotnet/docker-tools#1891

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