Add multi-arch Docker build & GHCR publishing

[Forne]

Summary

This PR addresses failures to run Yggdrasil on ARM systems. The root cause was the lack of ARM artifacts/images, which led to exec format error and similar issues.

What’s added:

• Dockerfile.multiarch — multi-stage Go build that correctly propagates GOOS/GOARCH for linux/amd64, linux/arm64, linux/armhf and linux/armel platform.
• entrypoint.sh - Introduced ENV ALLOW_IPV6_FORWARDING. When set to a truthy value (e.g., true), the entrypoint executes: sysctl -w net.ipv6.conf.all.forwarding=1.
• GitHub Action for multi-arch builds and publishing to GHCR — triggered via workflow_dispatch, push to master and release via tags (with docker semantic tags e.g. v0.5.12 → 0.5.12, 0.5, 0).

Example published images:
https://github.com/Forne/yggdrasil-go/pkgs/container/yggdrasil-go

Testing

✅ Ubuntu (24.04, amd64) — image runs correctly.
✅ macOS (Apple Silicon, arm64) — image runs correctly.
✅ MikroTik RouterOS (arm64) — image runs under the RouterOS container package.

[neilalexander]

Looks good, thanks!