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/Dockerfile-elastic

@@ -0,0 +1,10 @@
+FROM docker.elastic.co/elasticsearch/elasticsearch:5.6.1
+LABEL maintainer Sybren A. Stüvel <sybren@blender.studio>
+
+RUN elasticsearch-plugin remove --purge  x-pack
+
+ADD elasticsearch.yml jvm.options /usr/share/elasticsearch/config/
+
+USER root
+RUN chown -R elasticsearch:elasticsearch /usr/share/elasticsearch/config/
+USER elasticsearch