podman push without destination image

the destination image for podman push should be optional (if the destination
has already been tagged in).  the man page for podman push describes that it
should work this way.

Resolves: #645

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

Closes: #646
Approved by: mheon

cmd/podman/push.go

@@ -76,17 +76,25 @@ var (
 )
 
 func pushCmd(c *cli.Context) error {
-	var registryCreds *types.DockerAuthConfig
+	var (
+		registryCreds *types.DockerAuthConfig
+		destName      string
+	)
 
 	args := c.Args()
-	if len(args) < 2 {
-		return errors.New("podman push requires exactly 2 arguments")
+	srcName := args[0]
+	if len(args) == 0 || len(args) > 2 {
+		return errors.New("podman push requires at least one image name, and optionally a second to specify a different destination name")
+	}
+	switch len(args) {
+	case 1:
+		destName = args[0]
+	case 2:
+		destName = args[1]
 	}
 	if err := validateFlags(c, pushFlags); err != nil {
 		return err
 	}
-	srcName := args[0]
-	destName := args[1]
 
 	// --compress and --format can only be used for the "dir" transport
 	splitArg := strings.SplitN(destName, ":", 2)