Add multi-arch Docker build & GHCR publishing (#1278)

# 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](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.

contrib/docker/entrypoint.sh

@@ -9,5 +9,10 @@ if [ ! -f "$CONF_DIR/config.conf" ]; then
   yggdrasil --genconf > "$CONF_DIR/config.conf"
 fi
 
+if [ -n "$ALLOW_IPV6_FORWARDING" ]; then
+  echo "set sysctl -w net.ipv6.conf.all.forwarding=1"
+  sysctl -w net.ipv6.conf.all.forwarding=1
+fi
+
 yggdrasil --useconf < "$CONF_DIR/config.conf"
 exit $?