Use arm64 GitHub Actions runner

[shuuji3]

Currently, "build & push docker container" GitHub Actions takes ~20 minutes. This is mainly because Qemu is used to emulate an Arm VM to build arm64 container image. We can try a new Arm linux runner available in this August.

ref. arm64 hosted runners for public repositories are now generally available - GitHub Changelog - https://github.blog/changelog/2025-08-07-arm64-hosted-runners-for-public-repositories-are-now-generally-available/