Fix parallel startup of testcontainers

Update `TestcontainersLifecycleBeanPostProcessor` so that containers
can actually be started in parallel.

Prior to this commit, `initializeStartables` would collect beans
and in the process trigger the `postProcessAfterInitialization` method
on each bean. This would see that  `startablesInitialized` was `true`
and call `startableBean.start` directly. The result of this was that
beans were actually started sequentially and when the `start` method
was finally called it had nothing to do.

The updated code uses an enum rather than a boolean so that the
`postProcessAfterInitialization` method no longer attempts to start
beans unless `initializeStartables` has finished.

Fixes gh-38831

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

@@ -0,0 +1,75 @@
+/*
+ * 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.springframework.boot.test.system.CapturedOutput;
+import org.springframework.boot.test.system.OutputCaptureExtension;
+import org.springframework.boot.testcontainers.lifecycle.TestContainersParallelStartupIntegrationTests.ContainerConfig;
+import org.springframework.boot.testsupport.testcontainers.DisabledIfDockerUnavailable;
+import org.springframework.boot.testsupport.testcontainers.DockerImageNames;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.ContextConfiguration;
+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)
+@ContextConfiguration(classes = ContainerConfig.class)
+@TestPropertySource(properties = "spring.testcontainers.beans.startup=parallel")
+@DirtiesContext
+@DisabledIfDockerUnavailable
+@ExtendWith(OutputCaptureExtension.class)
+public class TestContainersParallelStartupIntegrationTests {
+
+	@Test
+	void startsInParallel(CapturedOutput out) {
+		assertThat(out).contains("-lifecycle-0").contains("-lifecycle-1").contains("-lifecycle-2");
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	static class ContainerConfig {
+
+		@Bean
+		static PostgreSQLContainer<?> container1() {
+			return new PostgreSQLContainer<>(DockerImageNames.postgresql());
+		}
+
+		@Bean
+		static PostgreSQLContainer<?> container2() {
+			return new PostgreSQLContainer<>(DockerImageNames.postgresql());
+		}
+
+		@Bean
+		static PostgreSQLContainer<?> container3() {
+			return new PostgreSQLContainer<>(DockerImageNames.postgresql());
+		}
+
+	}
+
+}