You need to commit the changes you make to the container and then run it. Try this:

sudo docker pull ubuntu

sudo docker run ubuntu apt-get install -y ping

Then get the container id using this command:

sudo docker ps -l

Commit changes to the container:

sudo docker commit <container_id> iman/ping 

Then run the container:

sudo docker run iman/ping ping www.google.com

This should work.