If container name is already unique, don't append "-0"

When using `--ensure-unique-name`, don't needlessly append `"-0"` if the
container name is already unique.

This is especially helpful with NixOps since when it deploys to a
container it uses `--ensure-unique-name`.  This means that the container
name will never match the deployment host due to the `"-0"`.  Having the
container name and the host name match isn't exactly a requirement, but
it's nice to have and a small change.