4e89a50a31
This fix tries to add a placeholder `.CreatedAt` for Go format template in `docker network ls --format`. While working on 29226, I noticed that it is not possible to display network's creation time in `docker network ls`, with or without `--format`. We are able to find the timestamp through `docker network inspect` though. However, as we allows networks to be pruned based on the timestamp (see 29226), showing the timestamp in `docker network ls --format` would be much useful now. This fix adds the `.CreatedAt` placeholder for `docker network ls --format`. The default output was not changed for `docker network ls --format`. A test case for unit tests has been added. Signed-off-by: Yong Tang <yong.tang.github@outlook.com>