feat: simplify Docker config file selection with convention over configuration

Implement automatic detection of custom_config.json for simpler Docker usage.
No environment variables or command-line arguments needed for most users.

Changes:
- entrypoint.sh: Add smart config detection (custom_config.json > CLI arg > default_config.json)
- docker-compose.yml: Add configs volume mount for editing without rebuilds
- docs/DOCKER.md: Update documentation with simplified workflow
- .gitignore: Add custom_config.json to prevent accidental commits

Usage:
  cp configs/default_config.json configs/custom_config.json
  nano configs/custom_config.json
  docker-compose up  # Automatically uses custom_config.json

Simplifies user experience by following convention over configuration principle.

entrypoint.sh

@@ -57,7 +57,19 @@ sleep 3
 
 # Step 4: Run trading agent with config file
 echo "🤖 Starting trading agent..."
-CONFIG_FILE="${1:-configs/default_config.json}"
+
+# Smart config selection: custom_config.json takes precedence if it exists
+if [ -f "configs/custom_config.json" ]; then
+    CONFIG_FILE="configs/custom_config.json"
+    echo "✅ Using custom configuration: configs/custom_config.json"
+elif [ -n "$1" ]; then
+    CONFIG_FILE="$1"
+    echo "✅ Using specified configuration: $CONFIG_FILE"
+else
+    CONFIG_FILE="configs/default_config.json"
+    echo "✅ Using default configuration: configs/default_config.json"
+fi
+
 python main.py "$CONFIG_FILE"
 
 # Cleanup on exit