Add multi-architecture build support to workflow

[rayjohnson]

Summary

• Refactors the push workflow to build and push multi-architecture images (amd64, arm64, armv7) using Docker Buildx
• Modernizes the workflow to use official GitHub Actions:
  • docker/setup-buildx-action for multi-arch build support
  • docker/login-action for secure registry authentication
  • docker/build-push-action for building and pushing images
• Adds proper OCI image labels (source, revision, created)
• Removes the artifact download/load pattern in favor of direct building

Benefits

• Enables multi-architecture support for wider platform compatibility
• Uses GitHub Actions best practices and official Docker actions
• Simplifies the workflow by combining build and push steps
• Adds standardized OCI metadata to images

Test plan

• Verify workflow runs successfully in GitHub Actions
• Confirm images are pushed to both GitHub Container Registry and Docker Hub
• Validate multi-arch images are available for all platforms (amd64, arm64, armv7)