docs: add Docker deployment design

Design for containerizing grist-mcp with:
- Multi-stage Dockerfile with Python 3.14
- SSE transport for remote server operation
- Docker Compose for local deployment
- Single adaptive CI workflow for Gitea and GitHub
- Semantic version tagging

docs/plans/2025-12-29-docker-deployment-design.md

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+# Docker Deployment Design
+
+## Overview
+
+Make grist-mcp deployable via Docker Compose with CI workflows for automated image builds on version tag pushes.
+
+## Requirements
+
+- Docker Compose for local deployment
+- Single CI workflow that works on both Gitea and GitHub
+- SSE transport (replacing stdio) for remote server operation
+- Port 3000 default, configurable via environment variable
+- Python 3.14
+- Semantic version tagging (1.2.3, 1.2, 1, latest)
+- Config mounted at runtime (not baked into image)
+
+## Files to Create
+
+### Dockerfile
+
+Multi-stage build:
+
+**Stage 1 (builder):**
+- Base: `python:3.14-slim`
+- Install uv
+- Copy `pyproject.toml` and `uv.lock`
+- Install dependencies
+
+**Stage 2 (runtime):**
+- Base: `python:3.14-slim`
+- Copy virtual environment from builder
+- Copy source code
+- Non-root user (`appuser`) for security
+- Expose port 3000
+- CMD: run server via uv
+
+### docker-compose.yaml
+
+```yaml
+services:
+  grist-mcp:
+    build: .
+    ports:
+      - "${PORT:-3000}:3000"
+    volumes:
+      - ./config.yaml:/app/config.yaml:ro
+    env_file:
+      - .env
+    restart: unless-stopped
+```
+
+### .env.example
+
+```
+PORT=3000
+GRIST_MCP_TOKEN=your-agent-token-here
+```
+
+### .github/workflows/build.yaml
+
+Single workflow that detects platform (Gitea vs GitHub) at runtime:
+
+- **Trigger:** Push of version tags (`v*.*.*`)
+- **Platform detection:** Check `GITEA_ACTIONS` environment variable
+- **Registry:**
+  - Gitea: `${{ github.server_url }}/${{ github.repository }}`
+  - GitHub: `ghcr.io/${{ github.repository }}`
+- **Authentication:**
+  - Gitea: `${{ secrets.REGISTRY_TOKEN }}`
+  - GitHub: `${{ secrets.GITHUB_TOKEN }}`
+- **Tags generated:** `1.2.3`, `1.2`, `1`, `latest`
+
+## Files to Modify
+
+### pyproject.toml
+
+Add dependencies:
+- `starlette` - ASGI framework for SSE
+- `uvicorn` - ASGI server
+- `sse-starlette` - SSE support
+
+### src/grist_mcp/main.py
+
+Replace stdio transport with SSE:
+
+1. Create Starlette ASGI app with routes:
+   - `GET /sse` - SSE connection endpoint
+   - `POST /messages` - Client message endpoint
+2. Run with uvicorn on configurable port (default 3000)
+3. Keep existing config/auth flow unchanged
+
+### .gitignore
+
+Ensure `.env` is ignored.
+
+## Implementation Order
+
+1. Update dependencies and main.py for SSE transport
+2. Create Dockerfile
+3. Create docker-compose.yaml and .env.example
+4. Create CI workflow
+5. Test locally with `docker compose up`
+
+## Secrets to Configure
+
+- **Gitea:** Create `REGISTRY_TOKEN` secret with registry push access
+- **GitHub:** Uses automatic `GITHUB_TOKEN`