Tooling issue: removed image restored after make

[jzstark]

When writing the book, sometimes I add images to the source code but later found that some images should be deleted. However, after running make, those deleted images come back to the source code.

This is because the old images are all copied to the container and then all copied back to host. The solution is simple, just make clean and then make should suffice. This is a just minor issue.