func NewOverlayFSFromEnv() (*OverlayFS, error) {
	layers := os.Getenv("KETOS_ROOTPATH_LAYERS")
	if len(layers) <= 0 {
		return NewDefaultOverlayFS(), nil
	}
	return NewOverlayFS(filepath.SplitList(layers)...)
}
NewOverlayFSFromEnv generate OverlayFS instance from environment variable
KETOS_ROOTPATH_LAYERS, default value is "/:/_ketos"
package rootpath
import (
	"github.com/pkg/errors"
	"io"
	"os"
	"path/filepath"
)
type OverlayFS struct {
	top    string
	lowers []string
}
func NewOverlayFS(layers ...string) (*OverlayFS, error) {
	absLayers := []string{}
	for _, layer := range layers {
		abs, err := filepath.Abs(layer)
		if err != nil {
			return nil, errors.Wrap(err, "calc abs path")
		}
		absLayers = append(absLayers, abs)
	}
	return NewSimpleOverlayFS(absLayers...), nil
}
func NewSimpleOverlayFS(layers ...string) *OverlayFS {
	return &OverlayFS{
		top:    layers[len(layers)-1],
		lowers: layers[:len(layers)-1],
	}
}