This happens because your docker containers are actually running on a virtual machine.
1st step would be to check what the IP of your docker machine (virtual machine) is :
$docker-machine ls
NAME ACTIVE DRIVER STATE URL SWARM
default * virtualbox Running tcp://192.168.99.100
Then run curl command which will show the default web site on your nginx web server inside the container
curl http://192.168.99.100:80