Split Docker builds into native amd64 + arm64 jobs

[mniedermaier]

Summary

• Fix arm64 build failure (libc-bin SIGSEGV under QEMU emulation)
• Replace single QEMU-emulated multi-arch job with parallel native builds
• Both architectures build simultaneously on native hardware

Architecture

build-amd64 (ubuntu-latest)  ──┐
                                ├──> create-manifests (merge into multi-arch tags)
build-arm64 (ubuntu-24.04-arm) ┘

Changes

• build-amd64: runs on x86 runner, pushes *-amd64 tags
• build-arm64: runs on native ARM runner, pushes *-arm64 tags
• create-manifests: merges platform images into latest, SHA, and version tags via docker buildx imagetools create

Test plan

• Verify both build jobs start on correct runner types
• Verify all 10 images build successfully on both architectures
• Verify multi-arch manifests are created with both platforms
• Verify docker pull on amd64 and arm64 hosts pulls correct image

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