refactor(ci): modernize docker build workflow

[Acet-Aminophen]

PR Checklist

• Required Checks
  • [v] Have you tested your changes?
    • ghcr test finished(https://github.com/Acet-Aminophen/RisuAI/actions/runs/22092057386, https://github.com/Acet-Aminophen/RisuAI/actions/runs/22091387395)
  • [v] Have you checked that it won't break any existing features?
    • Docker Image test finished

Summary

modernize docker build workflow

Related Issues

None

Changes

• Replace shell-based buildx build with docker/build-push-action@v6
• Update actions/checkout v3 → v6
• Update docker/setup-buildx-action v2 → v3
• Update docker/login-action v2 (SHA pinned) → v3
• Add docker/setup-qemu-action@v3 for ARM emulation(Not required, but recommended)
• Remove unused docker/metadata-action step

Impact

None. Just Refactoring.

Additional Notes

Have a nice day.