docs(readme): add cross-platform amd64 build guide and inline-env test run

README.md

@@ -97,6 +97,88 @@ git pull
 docker compose up -d --build
 ```
 
+### Cross-Platform Build (aarch64 → amd64)
+
+When building on an aarch64/ARM64 machine (Apple Silicon, ARM Windows WSL2, Raspberry Pi) for deployment to an x86_64/amd64 server:
+
+#### 1. Install buildx
+
+```bash
+# Download buildx for arm64
+BUILDX_VERSION=$(wget -qO- https://api.github.com/repos/docker/buildx/releases/latest | grep tag_name | head -1 | cut -d'"' -f4)
+wget "https://github.com/docker/buildx/releases/download/${BUILDX_VERSION}/buildx-${BUILDX_VERSION}.linux-arm64" -O ~/.docker/cli-plugins/docker-buildx
+chmod +x ~/.docker/cli-plugins/docker-buildx
+```
+
+#### 2. Register QEMU for amd64 emulation
+
+```bash
+docker run --privileged --rm tonistiigi/binfmt --install all
+```
+
+#### 3. Build for amd64
+
+```bash
+DOCKER_BUILDKIT=1 docker build --platform linux/amd64 -t legco_reranker:amd64 .
+```
+
+#### 4. Export and transfer to server
+
+```bash
+# Save image to tar file
+docker save legco_reranker:amd64 -o legco_reranker_amd64.tar
+
+# Compress (~762MB → ~250MB)
+gzip legco_reranker_amd64.tar
+
+# Transfer to server
+scp legco_reranker_amd64.tar.gz user@server:/path/
+
+# On the x86_64 server:
+gunzip legco_reranker_amd64.tar.gz
+docker load -i legco_reranker_amd64.tar
+
+# Run
+docker run -d --name legco -p 80:8000 --env-file backend/.env \
+  -v chroma_data:/app/chroma_db \
+  -v chunk_data:/app/document_chunk \
+  -v sqlite_data:/app/data \
+  legco_reranker:amd64
+```
+
+#### 5. Test run (local, port 8888)
+
+Before transferring to the server, test the amd64 image locally. Pass all config inline (no `--env-file`):
+
+```bash
+docker run -d --name legco_test -p 8888:8000 \
+  -e LLM_BASE_URL=https://openrouter.ai/api/v1 \
+  -e LLM_API_KEY=your_key_here \
+  -e LLM_MODEL_NAME=qwen/qwen3.6-35b-a3b \
+  -e LLM_TIMEOUT=60.0 \
+  -e EMBEDDING_MODEL=qwen/qwen3-embedding-4b \
+  -e EMBEDDING_BASE_URL=https://openrouter.ai/api/v1 \
+  -e EMBEDDING_API_KEY=your_key_here \
+  -e CHROMA_DB_PATH=./chroma_db \
+  -e CHUNK_SIZE=1000 \
+  -e CHUNK_OVERLAP=200 \
+  -e RETRIEVAL_N_RESULTS=10 \
+  -e RELEVANCE_THRESHOLD=7.0 \
+  -e PROMPTS_DB_PATH=./data/prompts.db \
+  -e HISTORY_DB_PATH=./data/history.db \
+  -e CORS_ORIGINS='["http://localhost:5173","http://localhost:3000"]' \
+  -v ~/woody/legco/data/chroma_db:/app/chroma_db \
+  -v ~/woody/legco/data/document_chunk:/app/document_chunk \
+  -v ~/woody/legco/data/data:/app/data \
+  legco_reranker:amd64
+
+# Verify
+curl http://localhost:8888/health
+
+# Clean up
+docker rm -f legco_test
+```
+
 ## Architecture
 
 ```