Don't set entire FQDN hostname to just container name

Below is more or less the same issue I describe in https://github.com/89luca89/distrobox/issues/62

**Describe the bug**
The container created by `toolbx` is setting the hostname in the container to `toolbox`. For example /etc/hostname is a value that is not actually the DNS name of any host on any network. See `man hostname` for all the assumptions made about and effects of setting /etc/hostname.

**Expected behaviour**
The FQDN hostname in the container should contain the FQDN of the host ~~be the same hostname as the host because they are sharing the network~~ .

In light of the resolution of 89luca89/distrobox#62, I'm striking out the suggestions I made initially. A summary conclusion is below at https://github.com/containers/toolbox/issues/969#issuecomment-1003551595.

~~On Debian there is a precedence for indicating the chroot in the command line prompt (chroot being a precursor to containers). To my knowledge there isn't an equivalent on Fedora based distros. Not knowing of an alternative I started using `osvirtalias` in [cnest](https://github.com/castedo/cnest).~~

~~I describe the situation in more detail here:
https://github.com/castedo/cnest/tree/master/prompt~~

~~So I can suggest doing similarly and set `osvirtalias` and/or `debian_chroot` to be the name of the container, and let the distro and container configuration determine the command line prompts however they are supposed to. And do this without incorrectly repurposing the hostname for a purpose other than what hostname is supposed to mean.~~

~~If you come across an alternative to `osvirtalias` please let me know. I don't want to start a precedent that is non-standard, but I do want to start a precedent other than `debian_chroot`. If it catches on, Debian distros probably want to use a name without chroot in it anyway.~~