macOS updates delete /var/root/.ssh

[lrworth]

I followed the instructions to add an entry to /var/root/.ssh/config, which worked until a macOS system update (to 12.4) deleted that file.

It's infeasible for me to reproduce this problem, but for now I've added the entry to /etc/ssh/ssh_config.d/nix-docker and will check this issue again for the next few system updates.

Please comment on this issue if:

• You have found the same problem, or
• You know whether /etc/ssh/ssh_config.d/nix-docker is a viable alternative to /var/root/.ssh/config

(If the latter is true, we should update the README)

[lrworth]

Also, I needed to add StrictHostKeyChecking accept-new for nix-docker, because the host was not listed in any known_hosts file. Unsure what is actually best practice here.