Separate build and publish jobs for each architecture

[Copilot]

This PR restructures the GitHub Actions workflow to separate build and publish operations into individual jobs for each architecture, improving parallelization and providing better control over CI/CD processes.

Changes Made

Previous Structure

• Single build job handled both architectures (linux/amd64, linux/arm64)
• Mixed build validation and publishing responsibilities
• Limited parallelization and harder to troubleshoot arch-specific issues

New Structure

Build Jobs (PRs, main, tags):

• build-amd64: Tests linux/amd64 builds without publishing
• build-arm64: Tests linux/arm64 builds without publishing

Publish Jobs (schedule, main):

• publish-amd64: Builds and publishes linux/amd64 images with -amd64 suffix
• publish-arm64: Builds and publishes linux/arm64 images with -arm64 suffix
• create-manifest: Combines both architectures into a multi-arch manifest

Benefits

• Better Parallelization: Each architecture builds independently, reducing overall CI/CD time
• Clear Separation: Build validation is separate from publishing, reducing complexity
• Architecture-Specific Control: Individual job control and troubleshooting per architecture
• Optimized Caching: Separate cache scopes for each architecture improve build performance
• Backward Compatibility: Final images still support both amd64 and arm64 architectures seamlessly

Validation

The workflow has been thoroughly tested to ensure:

• Build jobs run on PRs, main, and tags without pushing to registry
• Publish jobs run on schedule and main branch with registry publishing
• All platform configurations are correct (linux/amd64, linux/arm64)
• Multi-arch manifest creation works properly
• Job dependencies and execution conditions are validated

Documentation has been updated to explain the new CI/CD pipeline structure and its benefits.

Fixes #3.

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