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.

docs/DOCKER.md

@@ -112,17 +112,43 @@ docker-compose up
 
 ### Volume Mounts
 
-Docker Compose mounts three volumes:
+Docker Compose mounts three volumes for persistent data. By default, these are stored in the project directory:
 
 - `./data:/app/data` - Price data and trading records
 - `./logs:/app/logs` - MCP service logs
 - `./configs:/app/configs` - Configuration files (allows editing configs without rebuilding)
 
-Data persists across container restarts. To reset:
+### Custom Volume Location
+
+You can change where data is stored by setting `VOLUME_PATH` in your `.env` file:
+
+```bash
+# Store data in a different location
+VOLUME_PATH=/home/user/trading-data
+
+# Or use a relative path
+VOLUME_PATH=./volumes
+```
+
+This will store data in:
+- `/home/user/trading-data/data/`
+- `/home/user/trading-data/logs/`
+- `/home/user/trading-data/configs/`
+
+**Note:** The directory structure is automatically created. You'll need to copy your existing configs:
+```bash
+# After changing VOLUME_PATH
+mkdir -p /home/user/trading-data/configs
+cp configs/custom_config.json /home/user/trading-data/configs/
+```
+
+### Reset Data
+
+To reset all trading data:
 
 ```bash
 docker-compose down
-rm -rf data/agent_data/* logs/*
+rm -rf ${VOLUME_PATH:-.}/data/agent_data/* ${VOLUME_PATH:-.}/logs/*
 docker-compose up
 ```