Check for a mainClass in "run" task if there is one

If there is a run task with an explicit main it can be used
by the repackage task (assuming its own native setting is
not used).

Fixes gh-389

spring-boot-tools/spring-boot-gradle-plugin/src/main/groovy/org/springframework/boot/gradle/task/Repackage.java

@@ -23,6 +23,7 @@ import java.util.concurrent.TimeUnit;
 import org.gradle.api.Action;
 import org.gradle.api.DefaultTask;
 import org.gradle.api.Project;
+import org.gradle.api.tasks.JavaExec;
 import org.gradle.api.tasks.TaskAction;
 import org.gradle.api.tasks.bundling.Jar;
 import org.springframework.boot.gradle.SpringBootPluginExtension;
@@ -71,6 +72,11 @@ public class Repackage extends DefaultTask {
 		else if (extension.getCustomConfiguration() != null) {
 			libraries.setCustomConfigurationName(extension.getCustomConfiguration());
 		}
+		JavaExec runner = (JavaExec) project.getTasks().findByName("run");
+		if (runner!=null && mainClass==null) {
+			getLogger().info("Found main in run task: " + runner.getMain());
+			setMainClass(runner.getMain());
+		}
 		project.getTasks().withType(Jar.class, new RepackageAction(extension, libraries));
 	}