runconfig/opts: fix compilation issue

it was introduced with #20566 as a result of merge

Signed-off-by: Alexander Morozov <lk4d4@docker.com>
Upstream-commit: 93f57705110e196dca1cf2b2ce7d261ee97b9e4e
Component: engine

components/engine/runconfig/opts/parse.go

@@ -242,6 +242,7 @@ func Parse(cmd *flag.FlagSet, args []string) (*container.Config, *container.Host
 		entrypoint = strslice.StrSlice{*flEntrypoint}
 	}
 	// Validate if the given hostname is RFC 1123 (https://tools.ietf.org/html/rfc1123) compliant.
+	hostname := *flHostname
 	if hostname != "" {
 		matched, _ := regexp.MatchString("^(([[:alnum:]]|[[:alnum:]][[:alnum:]\\-]*[[:alnum:]])\\.)*([[:alnum:]]|[[:alnum:]][[:alnum:]\\-]*[[:alnum:]])$", hostname)
 		if !matched {