Implement GitHub Actions CI/CD pipeline

[noahwhite]

Description

Create automated build pipeline that triggers on releases, builds the sysext image, and prepares artifacts for upload.

Acceptance Criteria

• Create .github/workflows/build-and-publish.yml
• Configure workflow triggers:
  • Release creation/publication
  • Manual workflow dispatch with version parameter
• Implement build steps:
  • Build Docker image
  • Run build script inside container
  • Generate checksums (SHA256)
• Upload artifacts to GitHub release
• Run validation tests on built image:
  • Verify sysext structure
  • Verify binary is executable
  • Verify metadata is correct

Technical Notes

• Use Docker buildx for multi-platform builds if needed
• Cache Docker layers for faster builds
• Generate both alloy-{VERSION}-amd64.raw and alloy-{VERSION}.raw (compatibility)

Dependencies

• GHO-33: Build files must be migrated first

[linear]

GHO-34