Elasticsearch throws a warning because Transport SSL isn't enabled

Elasticsearch is printing the following warning because we are enabling X-Pack security (ON by default since v8.0.0) but don't have Transport SSL enabled (not on the main branch, at least):

{
  "@timestamp": "2022-02-11T19:15:57.791Z",
  "log.level": "WARN",
  "message": "Transport SSL must be enabled if security is enabled. Please set [xpack.security.transport.ssl.enabled] to [true] or disable security by setting [xpack.security.enabled] to [false]",
  "ecs.version": "1.2.0",
  "service.name": "ES_ECS",
  "event.dataset": "elasticsearch.server",
  "process.thread.name": "main",
  "log.logger": "org.elasticsearch.bootstrap.BootstrapChecks",
  "elasticsearch.node.name": "06c05d6947c9",
  "elasticsearch.cluster.name": "docker-cluster"
}

Right now, this is not preventing Elasticsearch from starting, but I'm worried that it may in a future release, especially due to the usage of the term "must" in the log message. We should keep our eyes open.

As a reminder, the tls branch has Transport SSL enabled by default. However, there were concerns about making this the default, mostly due to the fact that private keys are public until the user manually rotates them.

https://github.com/deviantony/docker-elk/blob/ba9c2f34c42f5e6f7f27fadc7b3b6cf374c1d4fc/elasticsearch/config/elasticsearch.yml#L19-L25

Start with elasticsearch/config/elasticsearch.yml at the referenced lines and compare the settings with the tls branch configuration. Read the linked discussion about private-key exposure before deciding whether Transport SSL or security defaults should change. Done means an agreed configuration approach that addresses the warning without introducing the documented key-management concern.