add build to main and as subcommand to image

add build as a local and remote-client command in the main
podman menu.  also ensure it is added to the image subcommands
as well.

Signed-off-by: baude <bbaude@redhat.com>

cmd/podman/main.go

@@ -30,6 +30,7 @@ var (
 // Commands that the remote and local client have
 // implemented.
 var mainCommands = []*cobra.Command{
+	_buildCommand,
 	_exportCommand,
 	_historyCommand,
 	_imagesCommand,