Feature suggestion: Add image preloading functionality.

[garfcat]

In the process of upgrading applications, Baetyl follows a strategy of first deleting the old POD and then launching a new POD. During this process, there is a service downtime, and most of the downtime is spent on downloading images. Therefore, adding an image preloading feature can significantly reduce the service downtime.

The upgraded application process can be modified as follows:

1. Successful image download,
2. Delete the old service,
3. Start the new service.