Support authentication to private docker registry

This commit adds the ability to configure Docker image registry
authentication credentials in the Maven and Gradle plugins. The
authentication credentials are passed to the Docker daemon with
all daemon API calls, and the daemon forwards the credentials to the
image registry when necessary. This makes it possible to use
builder and run images stored in a private Docker registry.

See gh-22972

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/DockerRegistryTests.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2012-2020 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.gradle.tasks.bundling;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.buildpack.platform.docker.configuration.DockerRegistryConfiguration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link DockerRegistry}.
+ *
+ * @author Wei Jiang
+ */
+public class DockerRegistryTests {
+
+	@Test
+	void getDockerRegistryConfiguration() {
+		DockerRegistry dockerRegistry = new DockerRegistry();
+		dockerRegistry.setUsername("username");
+		dockerRegistry.setPassword("password");
+		dockerRegistry.setEmail("mock@spring.com");
+		dockerRegistry.setUrl("http://mock.docker.registry");
+		DockerRegistryConfiguration dockerRegistryConfiguration = dockerRegistry.getDockerRegistryConfiguration();
+		assertThat(dockerRegistryConfiguration).isNotNull();
+		assertThat(dockerRegistryConfiguration.getUsername()).isEqualTo(dockerRegistry.getUsername());
+		assertThat(dockerRegistryConfiguration.getPassword()).isEqualTo(dockerRegistry.getPassword());
+		assertThat(dockerRegistryConfiguration.getEmail()).isEqualTo(dockerRegistry.getEmail());
+		assertThat(dockerRegistryConfiguration.getUrl()).isEqualTo(dockerRegistry.getUrl());
+	}
+
+}