Fix registry detection

ECR registry does not have a username component

src/docker.js

@@ -119,8 +119,9 @@ function ping(callback) {
 }
 
 function getRegistryConfig(image, callback) {
+    // https://github.com/docker/distribution/blob/release/2.7/reference/normalize.go#L62
     const parts = image.split('/');
-    if (parts.length === 2) return callback(null, null); // public docker registry
+    if (parts.length === 1 || (parts[0].match(/[.:]/) === null)) return callback(null, null); // public docker registry
 
     settings.getRegistryConfig(function (error, registryConfig) {
         if (error) return callback(error);