Like the course but need help

in order to cmplete the course i need help with first simple example

[email protected]:~ $ docker run docker/whalesay hello world
docker: Error response from daemon: OCI runtime create failed: container_linux.go:346: starting container process caused “exec: “hello”: executable file not found in $PATH”: unknown.
ERRO[0002] error waiting for container: context canceled

someone help with this , i am running on a pi/raspbian