Add documentation for DependsOn

Author: QuinnBast

docs/examples/junit4/generic/src/test/java/generic/DependsOnTest.java

@@ -0,0 +1,27 @@
+package generic;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.testcontainers.containers.GenericContainer;
+import org.testcontainers.utility.DockerImageName;
+
+import static org.junit.Assert.assertTrue;
+
+public class DependsOnTest {
+
+    @Rule
+    // dependsOn {
+    public GenericContainer<?> redis = new GenericContainer<>("redis:3.0.2")
+        .withExposedPorts(6379);
+    
+    public GenericContainer<?> nginx = new GenericContainer<>(DockerImageName.parse("nginx:1.9.4"))
+        .dependsOn(redis)
+        .withExposedPorts(80);
+    // }
+
+    @Test
+    public void testContainersAllStarted() {
+        assertTrue(redis.isRunning());
+        assertTrue(nginx.isRunning());
+    }
+}

docs/features/startup_and_waits.md

@@ -1,5 +1,13 @@
 # Waiting for containers to start or be ready
 
+## Depending on another container
+
+When using multiple containers, it is possible that one container that you are starting relies on the other to be started before it should be started itself. An example of this might be a database that needs to be started before your application container can link to it. You can tell a container that it depends on another container by using the `dependsOn` method:
+
+<!--codeinclude--> 
+[Depending on another container](../examples/junit4/generic/src/test/java/generic/DependsOnTest.java) inside_block:dependsOn
+<!--/codeinclude-->
+
 !!! info "Wait strategies vs Startup strategies"
 
     **Wait strategy:** is the container in a state that is useful for testing. This is generally approximated as 'can we talk to this container over the network'. However, there are quite a few variations and nuances.