feat: add Docker image quick start and GHCR publish workflow

[Razpines]

Summary

Closes #5.

Adds an official CPU-first Docker image path for the HTTP MCP server, a GHCR publish workflow, and a copy-paste Docker quick start in the README.

Changes

• Add Dockerfile for CPU-only / FTS-only unitydocs-mcp-http runtime
• Add config.docker.yaml (index.vector: none) for CPU-safe container defaults
• Add .dockerignore to keep build context small
• Add .github/workflows/docker-publish.yml to publish ghcr.io/razpines/unityrag
• Tighten README.md Docker onboarding section (run / wait / verify / connect)
• Add docs/docker_http_clients.md with Docker-hosted MCP client guidance (Codex verified, others planned)

Validation

Docker build

• docker build -t unityrag:dev . (passed)

Runtime validation (fresh volume)

• docker run -d --name unityrag-clean-test -p 8765:8765 -v unityrag-clean-test-data:/app/data unityrag:dev
• Observed first-run setup complete in container logs:
  • download -> unzip -> bake -> index -> server start
• Observed server startup log:
  • [unitydocs-mcp] starting streamable HTTP server on 0.0.0.0:8765/mcp
• curl -i http://localhost:8765/mcp returned 406 Not Acceptable (expected for plain GET without Accept: text/event-stream)

Persistence validation

• Restarted container with same named volume
• Confirmed immediate startup without re-download / re-bake / re-index

Clean-environment check

• Stopped local VS Code-spawned stdio unitydocs-mcp processes
• Deleted local repo .venv and data/
• Re-ran Docker with a fresh named volume and confirmed endpoint still responds after initial setup

Notes

• Docker scope remains CPU-only / FTS-only in this PR
• Follow-up issue for Unity version env var UX (breaking change) created separately: feat(config): require UNITY_DOCS_MCP_UNITY_VERSION and derive URL/paths from it #21