Expose `get_host` for use without a container.

[bentheiii]

When testing connectivity, we'd like to see what happens if a client starts up before a container-based service is available. For this reason, we'd like to be able to get the host name of a container before it actually starts, but the only way to get the host name (AFAICT) is through the get_host method of active containers.

I propose adding a new public function containers_host_name, with roughly the implementation:

pub async fn containers_host_name()->Result<Host>{
  Client::lazy_client().await?
    .docker_hostname().await
    .map_err(Into::into)
}

That will allow users to know the host names of containers before they start up.