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>
Upstream-commit: 0ad02c610880f7ccb96be418baa8667e41aba7a5
Component: engine
