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.

.github/workflows/docker.yml

@@ -0,0 +1,62 @@
+name: Docker Build
+
+on:
+  workflow_dispatch:
+  push:
+    branches: [ main, master ]
+    tags: [ 'v*' ]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  packages: write
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-docker:
+    name: Build Docker Package
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          fetch-depth: 0
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=tag
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        id: docker_build
+        with:
+          context: .
+          file: ./contrib/docker/Dockerfile.multiarch
+          platforms: linux/amd64,linux/arm64,linux/armhf,linux/armel
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          provenance: false