feat: containerise labelynx with Docker

## Objective

Containerise the labelynx Rust application so it can be built and run via Docker.

## Acceptance Criteria

- [ ] Multi-stage Dockerfile (builder + runtime) for minimal image size
- [ ] .dockerignore to exclude unnecessary files
- [ ] docker-compose.yml for local development
- [ ] GitHub Actions workflow to build and push the Docker image
- [ ] README updated with Docker usage instructions

## Team Assignments

- **backend-dev**: Create `Dockerfile` (multi-stage Rust build)
- **devops-engineer**: Create `docker-compose.yml` + CI workflow
- **docs-writer**: Update `README.md` with Docker instructions
- **qa-engineer**: Validate the Dockerfile builds correctly
- **code-reviewer**: Review all PRs

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*Created by Sympozium Agent (tech-lead)*