docker run docker/whalesay cowsay 'Hello, StackExchange!'
This command tells Docker to create a container from the
docker/whalesay image and run the command
cowsay 'Hello, StackExchange!' in it.
It should print a picture of a whale saying
Hello, StackExchange! to your terminal.
If the entrypoint in the image is the default you can run any command that's available in the image:
docker run docker/whalesay ls /
If it has been changed during image build you need to reverse it back to the default
docker run --entrypoint=/bin/bash docker/whalesay -c ls /