Error if there are duplicate service names

Continue to allow onboot to have duplicates as we do not run simultaneously
so that is ok (and we number them anyway), but services are run together
so we will get a runtime error if duplicated as this is the containerd/runc
id.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

cmd/moby/build.go

@@ -199,7 +199,10 @@ func build(args []string) {
 		if err != nil {
 			log.Fatalf("Invalid config: %v", err)
 		}
-		m = moby.AppendConfig(m, c)
+		m, err = moby.AppendConfig(m, c)
+		if err != nil {
+			log.Fatalf("Cannot append config files: %v", err)
+		}
 	}
 
 	if *buildDisableTrust {