Use single-node discovery type

Since we’re getting the following error on setup after adding `action.auto_create_index` maybe we need to enable `single_node` to work properly:

| + docker run --network elastic --rm appropriate/curl --max-time 120 --retry 120 --retry-delay 1 --retry-connrefused --show-error --silent http://es1:9200
| curl: (7) Failed to connect to es1 port 9200: Host is unreachable
[Rails/test]   ❌  Failure - Setup and start Elasticsearch

Author: mzagaja

entrypoint.sh

@@ -32,6 +32,7 @@ if [ "x${MAJOR_VERSION}" == 'x6' ]; then
     --env "cluster.routing.allocation.disk.threshold_enabled=false" \
     --env "bootstrap.memory_lock=true" \
     --env "ES_JAVA_OPTS=-Xms1g -Xmx1g" \
+    --env "discovery.type=single-node" \
     --env "xpack.security.enabled=false" \
     --env "xpack.license.self_generated.type=basic" \
     --ulimit nofile=65536:65536 \