Build custom images for ElasticSearch & Kibana

We can then remove X-Pack and control ElasticSearch's memory usage.

This also gives us the opportunity to let Kibana do its optimization when
we build the image, rather than every time the container is recreated.

docker/elastic/elasticsearch.yml

@@ -0,0 +1,7 @@
+cluster.name: "blender-cloud"
+network.host: 0.0.0.0
+
+# minimum_master_nodes need to be explicitly set when bound on a public IP
+# set to 1 to allow single node clusters
+# Details: https://github.com/elastic/elasticsearch/pull/17288
+discovery.zen.minimum_master_nodes: 1