fix: do not append trailing slash on empty server values

docs/deployment/registry-management.md

@@ -124,4 +124,3 @@ dokku registry:set node-js-app push-on-release
 # globally
 dokku registry:set --global push-on-release
 ```
-

plugins/registry/functions.go

@@ -17,11 +17,15 @@ func getRegistryServerForApp(appName string) string {
 		value = common.PropertyGet("registry", "--global", "server")
 	}
 
-	value = strings.TrimSuffix(value, "/") + "/"
-	if value == "hub.docker.com/" || value == "docker.io/" {
+	value = strings.TrimSuffix(value, "/")
+	if value == "hub.docker.com" || value == "docker.io" {
 		value = ""
 	}
 
+	if value != "" {
+		value = value + "/"
+	}
+
 	return value
 }