Many <none> images created after build a docker image

Docker image is composed of layers:

docker history aario/centos

Each row you see using command above is a separate layer. Also, with time, a number of "orphaned" layers will fill up your disk space. You can safely remove them with:

docker image prune

Those are intermediate image layers. I.e, all of the steps of your Dockerfile.

You can check what these images are made of with inspect command:

docker image inspect 2f???????

They do not use additional disk space, since they are part of your named image. They can save space when you have multiple Dockerfiles with the same steps (in the same order) because they work as cache.

A longer and more complete explanation can be found here.

Eventually, if you delete your image or change build steps, you could have dangling none images. Those can be deleted, as @Mike mentioned, with:

docker image prune