Docker rmi unable to remove images

As the comments on your question indicate, you have figured out that you need to use:

• docker rmi to remove images
• docker rm to remove containers

for a bit more background: there is a difference between:

1. A docker image,
2. A running container based off that image and
3. A stopped container based off that image

The stopped container is kept because running the container might have changed the file system in the container, you can then commit this stopped container to have a new image. (that's one way to create images, manually run the commands and commit the resulting container).

Creating images using docker build and Dockerfile, does the same thing, it runs the container executing the Dockerfile commands and commits the resulting images, only tagging the last image that was committed.