ElasticContainer: Update to version 5.6.10

Version 5.6.10 is now the latest on the 5.6 series. Upgrade to use that
in the tests.

There is no -oss version provided by elastic.co and the official 5.6.10
version includes authentication, resulting in test failures. Continue to
use the -alpine version from dockerhub.

Change-Id: Ic14f5701e1a1500667544dabcf6168955e34c74f
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 9ea93df..132d074 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
@@ -46,7 +46,7 @@
       case V2_4:
         return "elasticsearch:2.4.6-alpine";
       case V5_6:
-        return "elasticsearch:5.6.9-alpine";
+        return "elasticsearch:5.6.10-alpine";
       case V6_2:
         return "docker.elastic.co/elasticsearch/elasticsearch-oss:6.2.4";
     }