Allow dockerTest tasks to be run in parallel through configuration

Closes gh-44913

Author: wilkinsona

buildSrc/src/main/java/org/springframework/boot/build/test/DockerTestBuildService.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2024 the original author or authors.
+ * Copyright 2012-2025 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.
@@ -22,8 +22,10 @@
 import org.gradle.api.services.BuildServiceParameters;
 
 /**
- * Build service for Docker-based tests. Configured to only allow serial execution,
- * thereby ensuring that Docker-based tests do not run in parallel.
+ * Build service for Docker-based tests. The maximum number of {@code dockerTest} tasks
+ * that can run in parallel can be configured using
+ * {@code org.springframework.boot.dockertest.max-parallel-tasks}. By default, only a
+ * single {@code dockerTest} task will run at a time.
  *
  * @author Andy Wilkinson
  */
@@ -32,7 +34,16 @@ abstract class DockerTestBuildService implements BuildService<BuildServiceParame
 	static Provider<DockerTestBuildService> registerIfNecessary(Project project) {
 		return project.getGradle()
 			.getSharedServices()
-			.registerIfAbsent("dockerTest", DockerTestBuildService.class, (spec) -> spec.getMaxParallelUsages().set(1));
+			.registerIfAbsent("dockerTest", DockerTestBuildService.class,
+					(spec) -> spec.getMaxParallelUsages().set(maxParallelTasks(project)));
+	}
+
+	private static int maxParallelTasks(Project project) {
+		Object property = project.findProperty("org.springframework.boot.dockertest.max-parallel-tasks");
+		if (property == null) {
+			return 1;
+		}
+		return Integer.parseInt(property.toString());
 	}
 
 }