Migrate build files to new repository

[noahwhite]

Story Summary

As a platform maintainer, I want to migrate the existing build files from the ghost monorepo to the new alloy-sysext-build repository with improved documentation, so that the build tooling is centralized and well-documented for community use.

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✅ Acceptance Criteria

• Copy Dockerfile to new repo
• Copy build-alloy-sysext.sh to new repo
• Add comprehensive README with:
  • Purpose and overview
  • Build instructions
  • Local testing instructions
  • Version management approach
• Add example usage commands
• Document the systemd-sysext format and structure

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📝 Additional Context

• Design: Migrate files from /workspace/alloy-sysext-build/ in ghost repo
• Docs: Create comprehensive README for build process
• Related Issues/PRs: Depends on GHO-32 (repository creation)

Files to Migrate:

• /workspace/alloy-sysext-build/Dockerfile
• /workspace/alloy-sysext-build/build-alloy-sysext.sh

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📦 Definition of Ready

• Acceptance criteria defined
• No unresolved external dependencies
• Story is estimated
• Team has necessary skills and access
• Priority is clear
• Business value understood

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✅ Definition of Done

• All acceptance criteria met
• Unit/integration tests written & passing
• Peer-reviewed
• Docs updated (if applicable)
• Verified in staging (if needed)
• No critical bugs/regressions

[linear]

GHO-33