fix: add fallback default for HOST_DATA_DIR in docker-compose

[mehmetkr-31]

Summary

Adds a Compose-level fallback (${HOST_DATA_DIR:-./data}) for the execution service volume mount.

Problem

docker-compose.yml mounts chain data using ${HOST_DATA_DIR}:/data. If HOST_DATA_DIR is unset, Docker Compose expands the variable to an empty string and creates an anonymous volume. This leads to:

1. Chain data being stored in an opaque Docker-managed location.
2. Risk of accidental data loss when the volume is pruned.
3. Confusion for operators who expected data to persist to a local directory.

Solution

Use ${HOST_DATA_DIR:-./data} so that ./data is used as a safe fallback when the variable is missing. This aligns with the default already documented in .env.mainnet and .env.sepolia.

Impact

• Chain data is always persisted to a predictable local directory.
• No breaking change for operators who already set HOST_DATA_DIR.

Checklist

• Change is minimal and focused.
• Follows the same fallback pattern used elsewhere (e.g., ${NETWORK_ENV:-.env.mainnet}).
• No breaking changes.

[cb-heimdall]

🟡 Heimdall Review Status

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