Image Customizer: Validate HOME and USER env vars. (#9900)

The gpg command, and by extension, the tdnf command expects the USER and HOME environment variables to be valid for the OS they are running under (including under chroot). Since, the image customizer tool is typically run under `sudo` and since root is always a valid user, this generally isn't a problem. But this isn't true if `sudo -E` is used. And this can result in strange difficult to diganose errors in tdnf. So, this change verifies that `sudo -E` isn't being used.

toolkit/tools/pkg/imagecustomizerlib/imagecustomizer.go

@@ -230,6 +230,11 @@ func CustomizeImage(buildDir string, baseConfigPath string, config *imagecustomi
 		}
 	}()
 
+	err = checkEnvironmentVars()
+	if err != nil {
+		return err
+	}
+
 	// ensure build and output folders are created up front
 	err = os.MkdirAll(imageCustomizerParameters.buildDirAbs, os.ModePerm)
 	if err != nil {
@@ -897,3 +902,29 @@ func humanReadableUnitSizeAndName(size int64) (int64, string) {
 		return 1, "B"
 	}
 }
+
+func checkEnvironmentVars() error {
+	// Some commands, like tdnf (and gpg), require the USER and HOME environment variables to make sense in the OS they
+	// are running under. Since the image customization tool is pretty much always run under root/sudo, this will
+	// generally always be the case since root is always a valid user. However, this might not be true if the user
+	// decides to use `sudo -E` instead of just `sudo`. So, check for this to avoid the user running into confusing
+	// tdnf errors.
+	//
+	// In an ideal world, the USER, HOME, and PATH environment variables should be overridden whenever an external
+	// command is called under chroot. But such a change would be quite involved.
+	const (
+		rootHome = "/root"
+		rootUser = "root"
+	)
+
+	envHome := os.Getenv("HOME")
+	envUser := os.Getenv("USER")
+
+	if envHome != rootHome || envUser != rootUser {
+		return fmt.Errorf("tool should be run as root (e.g. by using sudo):\n"+
+			"HOME must be set to '%s' and USER must be set to '%s'",
+			rootHome, rootUser)
+	}
+
+	return nil
+}