This host has Docker but no images yet. Fill the image store and see how images and containers relate.

Look at the empty store, then find nginx on Docker Hub:

docker images
docker search nginx

Pull a pinned version and read the last line of the output:

Run hello-world twice. The first run pulls it:

docker run hello-world
docker run hello-world

Compare containers with images:

docker ps -a
docker images

Each docker run added a container. The image was downloaded once.