Add self-typing generics to ElasticsearchContainer (#963)

Fixes #962

Author: kiview

modules/elasticsearch/src/main/java/org/testcontainers/elasticsearch/ElasticsearchContainer.java

@@ -13,7 +13,7 @@
  * Represents an elasticsearch docker instance which exposes by default port 9200 and 9300 (transport.tcp.port)
  * The docker image is by default fetched from docker.elastic.co/elasticsearch/elasticsearch
  */
-public class ElasticsearchContainer extends GenericContainer {
+public class ElasticsearchContainer extends GenericContainer<ElasticsearchContainer> {
 
     /**
      * Elasticsearch Default HTTP port

modules/elasticsearch/src/test/java/org/testcontainers/elasticsearch/ElasticsearchContainerTest.java

@@ -46,7 +46,9 @@ public void stopRestClient() throws IOException {
 
     @Test
     public void elasticsearchDefaultTest() throws IOException {
-        try (ElasticsearchContainer container = new ElasticsearchContainer()){
+        try (ElasticsearchContainer container = new ElasticsearchContainer()
+            .withEnv("foo", "bar") // dummy env for compiler checking correct generics usage
+        ){
             container.start();
             Response response = getClient(container).performRequest(new Request("GET", "/"));
             assertThat(response.getStatusLine().getStatusCode(), is(200));