feat(deploy): add Docker Compose self-hosted deployment option

Add multi-platform Docker image build (amd64 + arm64) with GitHub Actions
CI that pushes to GHCR on every merge to main. Users can self-host with
a single `docker compose up -d` command.

- Dockerfile: multi-stage build with Next.js standalone output (~150-200MB)
- docker-compose.yml: one-command self-hosted deployment
- .github/workflows/docker.yml: CI workflow with QEMU cross-compilation
- next.config.ts: conditional `output: "standalone"` via BUILD_STANDALONE env
- README (zh/en/ja): restructure deploy section to include Docker option

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/docker.yml

@@ -0,0 +1,53 @@
+name: Build and push Docker image
+
+on:
+  push:
+    branches: [main]
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - 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@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest
+            type=sha,prefix=sha-
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max