islet_login: fix hostname resolution sudo msg

Addresses this warning:

 Enjoy yourself!
 Training materials are in /exercises

 sudo: unable to resolve host argus
 demo@argus:~$ ls

Docker 1.3 has a new option: --add-host.
Prior to 1.3 the loopback IP would be automatically set in the /etc/hosts file
for the given hostname of the container. This changed with 1.3 so now we should
pass it manually using --add-host.