ElasticContainer: Limit heap usage for test containers

By default Elasticsearch tells the JVM to use a heap with a minimum
and maximum size of 1 GB [1]. We don't need to use so much for the
tests; limit the heap to minimum/maximum 128 MB.

[1] https://www.elastic.co/guide/en/elasticsearch/reference/6.3/heap-size.html

Change-Id: I568ba5ef89f53516a50bf83619fb1ec748877d4e
diff --git a/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java b/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java
index de0eeab..95f7f1d 100644
--- a/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java
+++ b/gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/testing/ElasticContainer.java
@@ -30,6 +30,7 @@
     // See https://github.com/testcontainers/testcontainers-java/issues/343
     try {
       ElasticContainer<?> container = new ElasticContainer<>(version);
+      container.addEnv("ES_JAVA_OPTS", "-Xms128m -Xmx128m");
       container.start();
       return container;
     } catch (Throwable t) {