feat: add configurable volume path for persistent data

Add VOLUME_PATH environment variable to customize storage location for
all persistent data (data/, logs/, configs/ subdirectories).

Changes:
- .env.example: Add VOLUME_PATH variable with documentation
- docker-compose.yml: Use ${VOLUME_PATH:-.} for all volume mounts
- docs/DOCKER.md: Document custom volume location feature

Benefits:
- Store data outside project directory (e.g., on separate disk)
- Easier backup/restore (single directory)
- Clean separation of code and data
- Support for absolute or relative paths

Usage:
  # In .env file
  VOLUME_PATH=/home/user/trading-data

  # Results in mounts:
  /home/user/trading-data/data:/app/data
  /home/user/trading-data/logs:/app/logs
  /home/user/trading-data/configs:/app/configs

Defaults to current directory (.) if not set for backward compatibility.

.env.example

@@ -30,3 +30,9 @@ WEB_HTTP_PORT=8888
 
 # Agent Configuration
 AGENT_MAX_STEP=30
+
+# Data Volume Configuration
+# Base directory for all persistent data (will contain data/, logs/, configs/ subdirectories)
+# Use relative paths (./volumes) or absolute paths (/home/user/ai-trader-volumes)
+# Defaults to current directory (.) if not set
+VOLUME_PATH=.