Support for ephemeral host ports

[zhan9san]

Signed-off-by: Jack Zhang [email protected]

Related to #351

How I tested

$ cat docker-compose.yaml
version: '3.1'

services:
  nginx:
    image: nginx
    restart: always
    ports:
      - 80

$ sudo ~/src/github/nerdctl/_output/nerdctl --cni-path=/opt/cni/bin compose up
$ sudo nerdctl port 02
80/tcp -> 0.0.0.0:49153

$ sudo nerdctl run --cni-path=/opt/cni/bin --rm -p :80 nginx

[AkihiroSuda]

Thanks