1. Mount
首先创建目录：
mkdir -p ~/ov-demo/{lower,upper,work,merged}
cd ~/ov-demo
pwd在 lower 文件夹里面放两个文件:
echo "I am file A from lower" > lower/a.txt
echo "I am file B from lower" > lower/b.txt
ls lower/在挂载之前，merge、upper这些都是空的
ls merged/执行挂载
sudo mount -t overlay overlay -o \
lowerdir=$(pwd)/lower,upperdir=$(pwd)/upper,workdir=$(pwd)/work $(pwd)/merged验证：
ls merged/
ls upper/
2. 观察Copy-up
