Simplify support for pushing images to unauthenticated registries

This commit removes the requirement that `docker.publishRegistry`
configuration must be provided when an image is published by the Gradle
or Maven plugin.

Fixes gh-28844

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/docker/configuration/DockerConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -90,4 +90,9 @@ public final class DockerConfiguration {
 				new DockerRegistryUserAuthentication(username, password, url, email));
 	}
 
+	public DockerConfiguration withEmptyPublishRegistryAuthentication() {
+		return new DockerConfiguration(this.host, this.builderAuthentication,
+				new DockerRegistryUserAuthentication("", "", "", ""));
+	}
+
 }