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>
remove all containers
docker rm $(docker ps -a -q)