fix: implement per-client data directory fallback and update gitignore

[Kewe63]

Summary

Implements a default per-client fallback for HOST_DATA_DIR and updates .gitignore.

Problem

• Unset HOST_DATA_DIR led to anonymous volumes in Docker Compose.
• The default './data' directory (or any local data directory) was not ignored by git, risking accidental tracking of large chain datasets.

Solution

• Updated docker-compose.yml to use a client-specific fallback: ${HOST_DATA_DIR:-./${CLIENT:-reth}-data}.
• Added /data/ to .gitignore to cover the most common fallback path.

Verification

• Verified volume mapping correctly defaults to ./reth-data (or equivalent).
• Verified that local data directories are correctly ignored.

Impact

Prevents accidental repository bloat and ensures predictable data persistence.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
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