Add `authorized_keys_command.conf` with correct git directory

I couldn't push or pull from any repositories until I set `-git-dir` following the instructions in [knot-hosting](https://tangled.sh/@tangled.sh/core/blob/master/docs/knot-hosting.md#repositories), so this adds the `/etc/ssh/sshd_config.d/authorized_keys_command.conf` file to `rootfs`.