fix: correct .env defaults and document HOST_DATA_DIR usage

[AdekunleBamz]

Summary

This PR fixes an issue with the default .env file and improves documentation to prevent users from accidentally mixing client data directories.

Problem

The original .env file used shell variable interpolation syntax (${CLIENT:-geth}) which is not supported by Docker Compose's env_file parsing. Docker Compose reads .env files as simple KEY=VALUE pairs without shell expansion, meaning:

• HOST_DATA_DIR would resolve to the literal string ./${CLIENT}-data instead of ./geth-data
• This could cause confusing volume mount issues

Additionally, when users switch clients (e.g., from geth to reth), they need to also update HOST_DATA_DIR to avoid mixing data from different clients in the same directory.

Changes

.env

• Replace unsupported variable interpolation with static defaults (CLIENT=geth, HOST_DATA_DIR=./geth-data)
• Add trailing newline for POSIX compliance

README.md

• Add note explaining that HOST_DATA_DIR should be set when changing CLIENT
• Update example commands to show the correct usage pattern

Testing

• Verified .env now ends with a proper newline
• Changes are documentation/config only - no functional code changes

[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