main: Add a --version flag to show the build version

This embeds the version at build time and displays it when passed the
--version flag.

cmd/image-builder/main.go

@@ -25,8 +25,9 @@ import (
 )
 
 var (
-	osStdout io.Writer = os.Stdout
-	osStderr io.Writer = os.Stderr
+	osStdout     io.Writer = os.Stdout
+	osStderr     io.Writer = os.Stderr
+	BuildVersion           = "DEVEL"
 )
 
 // basenameFor returns the basename for directory and filenames
@@ -378,6 +379,7 @@ image-type and blueprint.
 Image-builder builds operating system images for a range of predefined
 operating systems like Fedora, CentOS and RHEL with easy customizations support.`,
 		SilenceErrors: true,
+		Version:       BuildVersion,
 	}
 	rootCmd.PersistentFlags().String("data-dir", "", `Override the default data directory for e.g. custom repositories/*.json data`)
 	rootCmd.PersistentFlags().StringArray("extra-repo", nil, `Add an extra repository during build (will *not* be gpg checked and not be part of the final image)`)