fix: align Docker working directory with reth for relative path compatibility

[rezzmah]

Summary

• Fixes relative path resolution issue in Docker containers by aligning working directory with standard reth
• Removes WORKDIR /app to match reth's Docker configuration where working directory defaults to /
• Resolves issue where relative paths like .tmp/genesis.json would incorrectly resolve to /app/.tmp/genesis.json instead of /.tmp/genesis.json

Problem

The issue occurred because:

1. bera-reth Docker image set WORKDIR /app
2. Standard reth Docker image has no explicit working directory (defaults to /)
3. When users mount volumes like -v $(pwd)/.tmp:/.tmp, the relative path .tmp/file.json resolves differently:
   • reth: .tmp/file.json → /.tmp/file.json ✅
   • bera-reth: .tmp/file.json → /app/.tmp/file.json ❌

Solution

Remove the WORKDIR /app line from Dockerfile to match reth's behavior exactly.

Test Plan

• Verified standard reth works with relative paths in Docker
• Confirmed bera-reth fails with same relative paths
• Tested fix using -w / override successfully
• Commit removes the problematic WORKDIR /app line

Closes #27