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/README.md

@@ -68,7 +68,15 @@ Blender Cloud expects the following files to exist:
 
 ## 7. ElasticSearch & kibana
 
-Kibana should be placed in read-only mode:
+ElasticSearch and Kibana run in our self-rolled images. This is needed because by default
+
+- ElasticSearch uses up to 2 GB of RAM, which is too much for our droplet, and
+- the Docker images contain the proprietary X-Pack plugin, which we don't want.
+
+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.
+
+Production Kibana should be placed in read-only mode:
 
 `curl -XPUT 'localhost:9200/.kibana/_settings' -d '{ "index.blocks.read_only" : true }'`