Why need 3 steps?

[superlbr]

I mean the image created is very large with so many steps, just one step not enough?

[dtabuenc]

The steps don't really increase the size of the image. The way docker works is images are layered so the next image re-uses the previous image storage. The different steps in the process allow to create intermediate image which you can use as a base for other oracle images with different configurations.