docker restart container failed: “already in use”, but there’s no more docker image

It is because

• you have used --name switch.
• container is stopped and not removed

You find it stopped

docker ps -a

You can simply start it using below command:

docker start webserver

EDIT: Alternatives
If you want to start the container with below command each time,

docker run -d -p 80:80 --name webserver nginx

then use one of the following:

method 1: use --rm switch i.e., container gets destroyed automatically as soon as it is stopped

docker run -d -p 80:80 --rm --name webserver nginx

method 2: remove it explicitly after stopping the container before starting the command that you are currently using.

docker stop <container name>
docker rm <container name>