Update /etc/hosts when linked container is restarted

Carry of #7092

closes #7092
fixes #6350

/cc @vieux
I'd reword to something like this:

> [Communication between containers](#between-containers). 
> On restart, Docker may assign a different IP address to the linked containers.
> To keep the link, Docker then updates the `ALIAS` entry in the recipient
> (i.e., child) container's `/etc/hosts` file.

Or:

> [Communication between containers](#between-containers). 
> Docker updates the `ALIAS` entry in the `/etc/hosts` file of the recipient
> containers (i.e., children) in order to keep the link since Docker may
> assign a different IP address to the linked containers on restart. 

Please note that "to keep the link", however, might not be the correct term to use here.