Add Networks placeholder to ps --format

Passing {{.Networks}} to the format parameter will prompt ps to
display all the networks the container is connected to.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

command/container/list.go

@@ -70,6 +70,13 @@ func (p *preProcessor) Size() bool {
 	return true
 }
 
+// Networks does nothing but return true.
+// It is needed to avoid the template check to fail as this field
+// doesn't exist in `types.Container`
+func (p *preProcessor) Networks() bool {
+	return true
+}
+
 func buildContainerListOptions(opts *psOptions) (*types.ContainerListOptions, error) {
 	options := &types.ContainerListOptions{
 		All:    opts.all,