ElasticSearch on Elastic Beanstalk

I'm trying to get ElasticSearch running in an Elastic Beanstalk environment. Using Docker image it's fairly straightforward to get one instance running in a load balanced environment. However, when I try to add more instances to the cluster, they fail to discover each other and every new one becomes a new_master.

My Dockerfile looks like following

FROM dockerfile/java:oracle-java8
RUN ... # Downloading and installing ElasticSearch
RUN /elasticsearch/bin/plugin install elasticsearch/elasticsearch-cloud-aws/2.5.0
VOLUME ["/data"]
ADD config/elasticsearch.yml /elasticsearch/config/elasticsearch.yml
WORKDIR /data
CMD ["/elasticsearch/bin/elasticsearch"]

EXPOSE 9200

And the configuration config/elasticsearch.yml looks like following:

cluster:
  name: elastic-env-dev
cloud:
  aws:
    region: ap-southeast-2
discovery:
  type: ec2
  ec2:
    tag:
      Name: elastic-env-dev
    ping_timeout: 120s

The name of the EB environment is elastic-env-dev.