Use default address if null address is given

Closes gh-42958

Author: mhalbritter

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

@@ -57,8 +57,11 @@ public class ResolvedDockerHost extends DockerHost {
 
 	@Override
 	public String getAddress() {
-		return super.getAddress().startsWith(UNIX_SOCKET_PREFIX)
-				? super.getAddress().substring(UNIX_SOCKET_PREFIX.length()) : super.getAddress();
+		String address = super.getAddress();
+		if (address == null) {
+			address = getDefaultAddress();
+		}
+		return address.startsWith(UNIX_SOCKET_PREFIX) ? address.substring(UNIX_SOCKET_PREFIX.length()) : address;
 	}
 
 	public boolean isRemote() {
@@ -100,7 +103,11 @@ static ResolvedDockerHost from(Environment environment, DockerHostConfiguration
 			DockerContext context = config.getContext();
 			return new ResolvedDockerHost(context.getDockerHost(), context.isTlsVerify(), context.getTlsPath());
 		}
-		return new ResolvedDockerHost(Platform.isWindows() ? WINDOWS_NAMED_PIPE_PATH : DOMAIN_SOCKET_PATH);
+		return new ResolvedDockerHost(getDefaultAddress());
+	}
+
+	private static String getDefaultAddress() {
+		return Platform.isWindows() ? WINDOWS_NAMED_PIPE_PATH : DOMAIN_SOCKET_PATH;
 	}
 
 	private static boolean isTrue(String value) {

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/test/java/org/springframework/boot/buildpack/platform/docker/configuration/ResolvedDockerHostTests.java

@@ -39,6 +39,7 @@
  * Tests for {@link ResolvedDockerHost}.
  *
  * @author Scott Frederick
+ * @author Moritz Halbritter
  */
 class ResolvedDockerHostTests {
 
@@ -64,6 +65,16 @@ void resolveWhenDockerHostIsNullReturnsWindowsDefault() throws Exception {
 		assertThat(dockerHost.getCertificatePath()).isNull();
 	}
 
+	@Test
+	@EnabledOnOs(OS.WINDOWS)
+	void resolveWhenUsingDefaultContextReturnsWindowsDefault() {
+		this.environment.put("DOCKER_CONTEXT", "default");
+		ResolvedDockerHost dockerHost = ResolvedDockerHost.from(this.environment::get, null);
+		assertThat(dockerHost.getAddress()).isEqualTo("//./pipe/docker_engine");
+		assertThat(dockerHost.isSecure()).isFalse();
+		assertThat(dockerHost.getCertificatePath()).isNull();
+	}
+
 	@Test
 	@DisabledOnOs(OS.WINDOWS)
 	void resolveWhenDockerHostAddressIsNullReturnsLinuxDefault() throws Exception {
@@ -75,6 +86,16 @@ void resolveWhenDockerHostAddressIsNullReturnsLinuxDefault() throws Exception {
 		assertThat(dockerHost.getCertificatePath()).isNull();
 	}
 
+	@Test
+	@DisabledOnOs(OS.WINDOWS)
+	void resolveWhenUsingDefaultContextReturnsLinuxDefault() {
+		this.environment.put("DOCKER_CONTEXT", "default");
+		ResolvedDockerHost dockerHost = ResolvedDockerHost.from(this.environment::get, null);
+		assertThat(dockerHost.getAddress()).isEqualTo("/var/run/docker.sock");
+		assertThat(dockerHost.isSecure()).isFalse();
+		assertThat(dockerHost.getCertificatePath()).isNull();
+	}
+
 	@Test
 	void resolveWhenDockerHostAddressIsLocalReturnsAddress(@TempDir Path tempDir) throws IOException {
 		String socketFilePath = Files.createTempFile(tempDir, "remote-transport", null).toAbsolutePath().toString();