Fix pusher flags for --config and --tarball

This was done for the digester in #1721 but not the pusher. This change
makes them consistent again.

container/go/cmd/pusher/pusher.go

@@ -38,8 +38,8 @@ import (
 
 var (
 	dst                 = flag.String("dst", "", "The destination location including repo and digest/tag of the docker image to push. Supports fully-qualified tag or digest references.")
-	imgTarball          = flag.String("tarball", "", "Path to the image tarball as generated by docker save.")
-	imgConfig           = flag.String("config", "", "Path to the image config.json. Required when --format is Docker.")
+	imgTarball          = flag.String("tarball", "", "Path to the image tarball as generated by docker save. Required if --config was not specified.")
+	imgConfig           = flag.String("config", "", "Path to the image config.json. Required if --tarball was not specified.")
 	baseManifest        = flag.String("manifest", "", "Path to the manifest of the base image. This should be the very first image in the chain of images and is only really required for windows images with a base image that has foreign layers.")
 	format              = flag.String("format", "", "The format of the uploaded image (Docker or OCI).")
 	clientConfigDir     = flag.String("client-config-dir", "", "The path to the directory where the client configuration files are located. Overiddes the value from DOCKER_CONFIG.")
@@ -81,8 +81,11 @@ func main() {
 	if *format == "" {
 		log.Fatalln("Required option -format was not specified.")
 	}
-	if *imgConfig == "" {
-		log.Fatalln("Option --config is required.")
+	if *imgTarball == "" && *imgConfig == "" {
+		log.Fatalln("Neither --tarball nor --config was specified.")
+	}
+	if *imgTarball != "" && *imgConfig != "" {
+		log.Fatalf("Both --tarball=%q & --config=%q were specified. Only one of them must be specified.", *imgTarball, *imgConfig)
 	}
 
 	// If the user provided a client config directory, ensure it's a valid