fix docker inspect for podman docker alias

diff --git a/dockerimg/dockerimg.go b/dockerimg/dockerimg.go
index f5ddd05..3ecb823 100644
--- a/dockerimg/dockerimg.go
+++ b/dockerimg/dockerimg.go
@@ -791,7 +791,7 @@
 
 	var curImgInitFilesHash string
 	if out, err := combinedOutput(ctx, "docker", "inspect", "--format", "{{json .Config.Labels}}", imgName); err != nil {
-		if strings.Contains(string(out), "No such object") {
+		if strings.Contains(strings.ToLower(string(out)), "no such object") {
 			// Image does not exist, continue and build it.
 			curImgInitFilesHash = ""
 		} else {