Add Docker Compose example for Umbrel/Start9

[Bortlesboat]

Summary

Create docker-compose configs optimized for Bitcoin node OS platforms (Umbrel, Start9, RaspiBlitz).

What's needed

• Docker Compose file that auto-connects to the platform's Bitcoin Core instance
• Environment variable mapping for each platform's RPC credentials
• Health check that verifies RPC connectivity
• Documentation for each platform

Platforms

• Umbrel (uses `APP_BITCOIN_NODE_IP`, `APP_BITCOIN_RPC_PORT`, etc.)
• Start9 (uses manifest.yaml + docker_entrypoint.sh)
• RaspiBlitz (standard RPC config)

Resources

• See `docs/marketing/umbrel-submission-guide.md` for Umbrel env var details

[Bortlesboat]

Grant-roadmap context: this deployment issue complements the Raspberry Pi / pruned-node self-hosting work. The goal is to make Satoshi API practical to run on low-cost, self-controlled infrastructure instead of relying on centralized Bitcoin data APIs.