docker: add healthcheck using WaitCatchup rpc

[tuxcanfly]

Overview

This PR adds a healthcheck to the celestia light node container on the testnet setup.

We use a hardcoded JWT token for ease of setup. This is not a security issue as this setup is meant for local testing only.

Fixes #48

Checklist

• [ ] New and updated code has appropriate documentation
• [ ] New and updated code has new and/or updated testing
• [ ] Required CI checks are passing
• [ ] Visual proof for any user facing features like CLI or documentation updates
• [ ] Linked issues closed with keywords