Don't start containers imported via @ImportTestcontainers

Remove early start of containers imported via `@ImportTestcontainers`
so that parallel startup can happen.

Fixes gh-38831

spring-boot-project/spring-boot-testcontainers/src/main/java/org/springframework/boot/testcontainers/context/ContainerFieldsImporter.java

@@ -22,7 +22,6 @@ import java.util.ArrayList;
 import java.util.List;
 
 import org.testcontainers.containers.Container;
-import org.testcontainers.lifecycle.Startable;
 
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.util.Assert;
@@ -39,9 +38,6 @@ class ContainerFieldsImporter {
 		for (Field field : getContainerFields(definitionClass)) {
 			assertValid(field);
 			Container<?> container = getContainer(field);
-			if (container instanceof Startable startable) {
-				startable.start();
-			}
 			registerBeanDefinition(registry, field, container);
 		}
 	}

spring-boot-project/spring-boot-testcontainers/src/test/java/org/springframework/boot/testcontainers/lifecycle/TestContainersParallelStartupWithImportTestcontainersIntegrationTests.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2012-2023 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.testcontainers.lifecycle;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.testcontainers.containers.PostgreSQLContainer;
+import org.testcontainers.junit.jupiter.Container;
+
+import org.springframework.boot.test.system.CapturedOutput;
+import org.springframework.boot.test.system.OutputCaptureExtension;
+import org.springframework.boot.testcontainers.context.ImportTestcontainers;
+import org.springframework.boot.testcontainers.lifecycle.TestContainersParallelStartupWithImportTestcontainersIntegrationTests.Containers;
+import org.springframework.boot.testsupport.testcontainers.DisabledIfDockerUnavailable;
+import org.springframework.boot.testsupport.testcontainers.DockerImageNames;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.TestPropertySource;
+import org.springframework.test.context.junit.jupiter.SpringExtension;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration test for parallel startup.
+ *
+ * @author Phillip Webb
+ */
+@ExtendWith(SpringExtension.class)
+@TestPropertySource(properties = "spring.testcontainers.beans.startup=parallel")
+@DirtiesContext
+@DisabledIfDockerUnavailable
+@ExtendWith(OutputCaptureExtension.class)
+@ImportTestcontainers(Containers.class)
+public class TestContainersParallelStartupWithImportTestcontainersIntegrationTests {
+
+	@Test
+	void startsInParallel(CapturedOutput out) {
+		assertThat(out).contains("-lifecycle-0").contains("-lifecycle-1").contains("-lifecycle-2");
+	}
+
+	static class Containers {
+
+		@Container
+		static PostgreSQLContainer<?> container1 = new PostgreSQLContainer<>(DockerImageNames.postgresql());
+
+		@Container
+		static PostgreSQLContainer<?> container2 = new PostgreSQLContainer<>(DockerImageNames.postgresql());
+
+		@Container
+		static PostgreSQLContainer<?> container3 = new PostgreSQLContainer<>(DockerImageNames.postgresql());
+
+	}
+
+}