ci: add Docker Hub publishing for netrunner

- Added docker-publish.yml workflow
- Publishes to docker.io/luxfi/netrunner and ghcr.io/luxfi/netrunner
- Multi-arch: linux/amd64 + linux/arm64

Usage:
  docker run --rm luxfi/netrunner --help

.github/workflows/docker-publish.yml

@@ -0,0 +1,74 @@
+name: Build and Publish Docker Image
+
+on:
+  push:
+    branches:
+      - main
+      - master
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - main
+      - master
+  workflow_dispatch:
+
+env:
+  GHCR_REGISTRY: ghcr.io
+  DOCKERHUB_REGISTRY: docker.io
+  IMAGE_NAME: luxfi/netrunner
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.GHCR_REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.DOCKERHUB_REGISTRY }}
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASS }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: |
+            ${{ env.GHCR_REGISTRY }}/${{ env.IMAGE_NAME }}
+            ${{ env.DOCKERHUB_REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha,prefix={{branch}}-
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          platforms: linux/amd64,linux/arm64