fix: add HOST_IP support to nethermind entrypoint

[Kewe63]

Summary

Adds HOST_IP support to the Nethermind entrypoint for improved P2P peer discovery behind NAT.

Problem

Nethermind nodes behind NAT lacked a way to advertise their external IP, leading to poor peer connectivity.

Solution

• Added safe initialization of the HOST_IP variable to maintain 'set -u' compatibility.
• Implemented the --Network.ExternalIp flag when HOST_IP is provided.

Verification

• Verified the flag is correctly appended during execution when HOST_IP is set.

Impact

Aligns Nethermind's NAT handling with other clients and improves connectivity for NAT operators.

[cb-heimdall]

🟡 Heimdall Review Status

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Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2