3b: Docker image

[valiantone]

Summary

Publish a Docker image for zero-install adoption:
docker run -p 8711:8711 -v ./data:/data knowguard/hotmem

Scope

• Dockerfile (Alpine-based, multi-arch amd64 + arm64)
• GitHub Actions workflow to build and push on release
• Publish to Docker Hub (knowguard/hotmem) and GHCR
• Default command: hotmem serve --mount /data --host 0.0.0.0

Acceptance Criteria

• Single docker run command starts a working HotMem server
• Volume mount persists data across container restarts
• Image size under 100MB
• Works on both amd64 and arm64