Feature Request: Add Dockerfile for easier local development setup

### Describe the solution you'd like

Hi team 👋

I’d like to suggest adding a Dockerfile (and possibly a docker-compose.yml) to OnchainKit to make local development and onboarding smoother for contributors.

Why this feature would be useful

Ensures a consistent development environment across systems (Node 20, pnpm 10, etc.).

Makes it much easier for new contributors to get started — no need for manual setup.

Useful for testing and CI since everything runs in a reproducible container.

Optional: could include live-reload support with Docker volumes for active development.

### Describe alternatives you've considered.

Use Node 20 as the base image (matches current requirements).

Copy package.json / pnpm-lock.yaml, install dependencies with pnpm.

Copy the full repo and expose port 3000 for the playground.

Add .dockerignore to reduce image size.

Update README.md with an example of running via Docker.