Store instance data in docker volume

config/elasticsearch.yml to store data, logs, plugins, and work(tmp) dirs in /data
ADD config/elasticsearch.yml /elasticsearch/config/elasticsearch.yml

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Currently, `docker run`ing the **dockerfile/elasticsearch** container doesn't store the Elasticsearch instance data on a docker volume (as explained in the README). This can cause unexpected data loss (and lack of data reusability with --volumes-from) for users that fail to configure the container volume (& ES config) per the README.

This change adds a short config that specifies the data, logs, plugins, and "work"(tmp) directories should reside on the docker volume by default.