VOL-595 - container image name consistency and general cleanup

Change-Id: Iccac1ccba61537cefa046118df139196e9e87713
diff --git a/compose/docker-compose-elasticsearch.yml b/compose/docker-compose-elasticsearch.yml
index 0b190b2..97f6176 100644
--- a/compose/docker-compose-elasticsearch.yml
+++ b/compose/docker-compose-elasticsearch.yml
@@ -2,7 +2,7 @@
 
 services:
   elasticsearch:
-    image: docker.elastic.co/elasticsearch/elasticsearch:5.6.0
+    image: "${REGISTRY}docker.elastic.co/elasticsearch/elasticsearch:${ELASTIC_TAG:-5.6.0}"
     environment:
       - transport.host=localhost
       - transport.tcp.port=9300