Use a default for HOST_NETWORK_ADDRESS to hide warnings

[dhruvkb]

Description

Docker Compose performs all env var interpolation in the full docker-compose.yml file regardless of which services are being started. When using a recipe like api/init (that does not refer to the root justfile), the vars are not set and Docker Composes raises warnings for missing HOST_NETWORK_ADDRESS.

This PR

• sets a default for the HOST_NETWORK_ADDRESS env var to suppress warnings raised by Docker Compose
• moves the env var export in the root to the Docker section

Testing Instructions

You should no longer see errors like the following when running api/init.

Checklist

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• I added or updated tests for the changes I made (if applicable).
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• I ran the DAG documentation generator (if applicable).

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[AetherUnbound]

Thank you for addressing this!