feat(ci): add multi-arch Docker build (amd64 + arm64)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pikespeak

.github/workflows/publish.yml

@@ -38,6 +38,10 @@ jobs:
       packages: write
     steps:
       - uses: actions/checkout@v6
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
       - name: Log in to GHCR
         uses: docker/login-action@v4
         with:
@@ -51,6 +55,7 @@ jobs:
         uses: docker/build-push-action@v7
         with:
           context: .
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: |
             ghcr.io/ottersight/cli:${{ steps.version.outputs.VERSION }}