Manual about using of ssh-agent instead of copying ssh private keys.

[WoZ]

Hi, @m-bers .

I want to present an improvement for docker-virt-manager. I tried to use this docker image and faced problems while using ssh keys with passphrases. You must enter passphrase a lot of times using ttyd approach (creation of a VM, creating connections, during an installation of VMs). It's pretty annoying, so I added manual how to act in a proper way with ssh-agent and passing $SSH_AUTH_SOCK to the container.

Fill free to comment and have a good day.