docs: document HOST_IP and HOST_DATA_DIR in env files

[Kewe63]

Summary

Adds documentation for networking and storage settings in environment files and fixes a HOST_IP clobbering bug in Geth.

Problem

• Advanced settings like HOST_IP and HOST_DATA_DIR were undocumented, leading to confusion for operators behind NAT.
• The Geth entrypoint was hardcoding HOST_IP="", which overrode any value set in environment files.
• The relationship between docker-compose.yml and the network-specific .env files (like .env.mainnet) was not clear regarding interpolation.

Solution

• Added documented examples to the root .env and network-specific .env files.
• Fixed the Geth entrypoint to use HOST_IP="${HOST_IP:-}" to preserve environment overrides.
• Added clarifying notes about where HOST_DATA_DIR must be set to take effect in docker-compose.

Verification

• Verified that Geth now correctly picks up HOST_IP from the environment.
• Read through the updated documentation for clarity.

Impact

Improves operator experience and discoverability of advanced configuration options.

[cb-heimdall]

🟡 Heimdall Review Status

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