Add `.CreatedAt` placeholder for `docker network ls --format`

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>

cli/command/formatter/network.go

@@ -115,3 +115,8 @@ func (c *networkContext) Label(name string) string {
 	}
 	return c.n.Labels[name]
 }
+
+func (c *networkContext) CreatedAt() string {
+	c.AddHeader(createdAtHeader)
+	return c.n.Created.String()
+}

cli/command/formatter/network_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/pkg/stringid"
@@ -142,14 +143,24 @@ network_id: networkID2
 			Context{Format: NewNetworkFormat("{{.Name}}", false)},
 			`foobar_baz
 foobar_bar
+`,
+		},
+		// Custom Format with CreatedAt
+		{
+			Context{Format: NewNetworkFormat("{{.Name}} {{.CreatedAt}}", false)},
+			`foobar_baz 2016-01-01 00:00:00 +0000 UTC
+foobar_bar 2017-01-01 00:00:00 +0000 UTC
 `,
 		},
 	}
 
+	timestamp1, _ := time.Parse("2006-01-02", "2016-01-01")
+	timestamp2, _ := time.Parse("2006-01-02", "2017-01-01")
+
 	for _, testcase := range cases {
 		networks := []types.NetworkResource{
-			{ID: "networkID1", Name: "foobar_baz", Driver: "foo", Scope: "local"},
-			{ID: "networkID2", Name: "foobar_bar", Driver: "bar", Scope: "local"},
+			{ID: "networkID1", Name: "foobar_baz", Driver: "foo", Scope: "local", Created: timestamp1},
+			{ID: "networkID2", Name: "foobar_bar", Driver: "bar", Scope: "local", Created: timestamp2},
 		}
 		out := bytes.NewBufferString("")
 		testcase.context.Output = out
@@ -168,8 +179,8 @@ func TestNetworkContextWriteJSON(t *testing.T) {
 		{ID: "networkID2", Name: "foobar_bar"},
 	}
 	expectedJSONs := []map[string]interface{}{
-		{"Driver": "", "ID": "networkID1", "IPv6": "false", "Internal": "false", "Labels": "", "Name": "foobar_baz", "Scope": ""},
-		{"Driver": "", "ID": "networkID2", "IPv6": "false", "Internal": "false", "Labels": "", "Name": "foobar_bar", "Scope": ""},
+		{"Driver": "", "ID": "networkID1", "IPv6": "false", "Internal": "false", "Labels": "", "Name": "foobar_baz", "Scope": "", "CreatedAt": "0001-01-01 00:00:00 +0000 UTC"},
+		{"Driver": "", "ID": "networkID2", "IPv6": "false", "Internal": "false", "Labels": "", "Name": "foobar_bar", "Scope": "", "CreatedAt": "0001-01-01 00:00:00 +0000 UTC"},
 	}
 
 	out := bytes.NewBufferString("")

docs/reference/commandline/network_ls.md

@@ -182,16 +182,17 @@ using a Go template.
 
 Valid placeholders for the Go template are listed below:
 
-Placeholder | Description
-------------|------------------------------------------------------------------------------------------
-`.ID`       | Network ID
-`.Name`     | Network name
-`.Driver`   | Network driver
-`.Scope`    | Network scope (local, global)
-`.IPv6`     | Whether IPv6 is enabled on the network or not.
-`.Internal` | Whether the network is internal or not.
-`.Labels`   | All labels assigned to the network.
-`.Label`    | Value of a specific label for this network. For example `{{.Label "project.version"}}`
+Placeholder  | Description
+-------------|------------------------------------------------------------------------------------------
+`.ID`        | Network ID
+`.Name`      | Network name
+`.Driver`    | Network driver
+`.Scope`     | Network scope (local, global)
+`.IPv6`      | Whether IPv6 is enabled on the network or not.
+`.Internal`  | Whether the network is internal or not.
+`.Labels`    | All labels assigned to the network.
+`.Label`     | Value of a specific label for this network. For example `{{.Label "project.version"}}`
+`.CreatedAt` | Time when the network was created
 
 When using the `--format` option, the `network ls` command will either
 output the data exactly as the template declares or, when using the