feat: Add ARM64 image builds

[chasenio]

Summary

• publish Futu and moomoo images for linux/amd64 and linux/arm64
• run the upstream x86-64 OpenD binary through bundled QEMU in the ARM64 image
• keep the AMD64 runtime native and copy only the required emulator/runtime files into ARM64
• document ARM64 build usage, limitations, and the deferred helper-process wrapper proposal
• update Docker Actions to currently supported major versions

Validation

• GitHub Actions build and push succeeded: https://github.com/chasenlab/opend/actions/runs/30145150288
• both GHCR OCI indexes contain linux/amd64 and linux/arm64 manifests
• Futu and moomoo ARM64 images ran -help successfully on an ARM64 host
• default ARM64 startup loaded configuration and started the API listener; the Futu listener accepted a local connection
• Futu and moomoo AMD64 images ran natively on an x86-64 Mac mini and reached configuration/API startup
• actionlint, Dockerfile checks, shell syntax checks, and git diff --check passed

Known limitations

Futu currently provides only x86-64 Linux binaries, so ARM64 uses QEMU user-mode emulation and may be slower. WebSocket, updater, and monitor/watchdog helper-process behavior still needs validation on a host without x86-64 binfmt_misc; the proposed wrapper fallback is documented but intentionally not implemented in this PR.