fix: RPC server unreachable from outside Docker container

Problem

The RPC server is hardcoded to 127.0.0.1 in main.py:659:

await rpc_server.start(host="127.0.0.1", port=rpc_port)

The Dockerfile correctly passes --host 0.0.0.0 for the P2P
server via CMD, but the RPC server ignores the --host flag
entirely — it is hardcoded in Python code.

This creates a contradiction:

Steps to Reproduce

docker run -p 8545:8545 -p 9000:9000 ghcr.io/stabilitynexus/minichain
curl http://localhost:8545
# connection refused

Expected Behaviour

RPC server should be reachable from outside the container
when -p 8545:8545 is passed.

Proposed Fix

Three small changes:

1. Add --rpc-host CLI argument to main.py (default
   127.0.0.1 for bare metal safety)
2. Use args.rpc_host instead of hardcoded "127.0.0.1"
   on line 659
3. Pass --rpc-host 0.0.0.0 in Dockerfile CMD and add
   EXPOSE 8545

Start in main.py around line 659 and inspect the existing CLI argument handling, then review the Dockerfile CMD, HEALTHCHECK, and EXPOSE declarations. Reproduce the issue with the provided docker run and curl commands. Done means the RPC host is configurable, Docker binds it externally, port 8545 is declared, and the container remains healthy.