Don't call Startable.start() for already started containers

Add a new `TestcontainersStartup.start` static method and update the
existing start methods so that `Startable.start()` is only called when
the container is not already running.

Prior to this commit, we assumed that `Startable.start()` calls were
idempotent and could be safely made multiple times. Whilst this appears
to be true for stock `GenericContainer` based startables, users may have
their own `start()` method that does not expect to be called multiple
times.

The implemented detection logic will not be applied if a `Startable`
is not also a `Container`. In these cases, the implementation will need
to deal directly with multiple `start()` calls.

Fixed gh-43253

spring-boot-project/spring-boot-testcontainers/src/dockerTest/java/org/springframework/boot/testcontainers/service/connection/ServiceConnectionStartsConnectionOnceIntegrationTest.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2012-2024 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.service.connection;
+
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.junit.jupiter.api.Test;
+import org.testcontainers.containers.PostgreSQLContainer;
+import org.testcontainers.junit.jupiter.Container;
+import org.testcontainers.junit.jupiter.Testcontainers;
+import org.testcontainers.utility.DockerImageName;
+
+import org.springframework.boot.autoconfigure.ImportAutoConfiguration;
+import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
+import org.springframework.boot.testsupport.container.TestImage;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Integration tests to ensure containers are started only once.
+ *
+ * @author Phillip Webb
+ */
+@SpringJUnitConfig
+@Testcontainers(disabledWithoutDocker = true)
+class ServiceConnectionStartsConnectionOnceIntegrationTest {
+
+	@Container
+	@ServiceConnection
+	static final StartCountingPostgreSQLContainer postgres = TestImage
+		.container(StartCountingPostgreSQLContainer.class);
+
+	@Test
+	void startedOnlyOnce() {
+		assertThat(postgres.startCount.get()).isOne();
+	}
+
+	@Configuration(proxyBeanMethods = false)
+	@ImportAutoConfiguration(DataSourceAutoConfiguration.class)
+	static class TestConfiguration {
+
+	}
+
+	static class StartCountingPostgreSQLContainer extends PostgreSQLContainer<StartCountingPostgreSQLContainer> {
+
+		final AtomicInteger startCount = new AtomicInteger();
+
+		StartCountingPostgreSQLContainer(DockerImageName dockerImageName) {
+			super(dockerImageName);
+		}
+
+		@Override
+		public void start() {
+			this.startCount.incrementAndGet();
+			super.start();
+		}
+
+	}
+
+}