Logstash output to Elasticsearch SSL certificate – Docker

I have the following docker containers

docker create 
--name=elasticsearch 
--restart=always 
--network=infrastructure_network 
--network-alias=elasticsearch 
-e TZ=Etc/UTC 
-e discovery.type=single-node 
-e "ES_JAVA_OPTS=-Xms6g -Xmx6g" 
-e ELASTIC_PASSWORD="foobar" 
-p 9200:9200 
-p 9300:9300 
-v elasticsearch:/usr/share/elasticsearch 
elasticsearch:8.0.0

docker create 
--name=logstash 
--restart=always 
--network=infrastructure_network 
--network-alias=logstash 
-e TZ=Etc/UTC 
-p 5040:5040 
-p 8514:8514/udp 
-v logstash:/usr/share/logstash/ 
-v elasticsearch:/elastic/ 
logstash:8.0.0

The containers start perfectly fine, but.. when I set the logstash output as follows:

input 
{
  tcp 
  {
    port => 8514
    type => syslog
  }
}

filter 
{
  if [type] == "syslog" 
  {
    grok 
    {
      match => { "message" => "%{SYSLOGTIMESTAMP:syslog_timestamp} %{SYSLOGHOST:syslog_hostname} %{DATA:syslog_program}(?:[%{POSINT:syslog_pid}])?: %{GREEDYDATA:syslog_message}" }
      add_field => [ "received_at", "%{@timestamp}" ]
      add_field => [ "received_from", "%{host}" ]
    }
    date 
    {
      match => [ "syslog_timestamp", "MMM  d HH:mm:ss.SSS", "MMM dd HH:mm:ss.SSS" ]
      timezone => "UTC"
    }
  }
}

output 
{
    elasticsearch 
    {
        hosts => ['https://elasticsearch:9200']
        cacert => '/elastic/config/certs/http_ca.crt'
        ssl_certificate_verification => false
        user => "elastic"
        password => "foobar"
        index => "syslog"
        ilm_enabled => false
    }

    stdout { codec => rubydebug }
}

I get the following error in the logstash logs…

Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

[2022-03-02T03:27:04,496][WARN ][logstash.outputs.elasticsearch][main] Attempted to resurrect connection to dead ES instance, but got an error {:url=>"https://elastic:xxxxxx@elasticsearch:9200/", :exception=>LogStash::Outputs::ElasticSearch::HttpClient::Pool::HostUnreachableError, :message=>"Elasticsearch Unreachable: [https://elasticsearch:9200/][Manticore::ClientProtocolException] PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target"}

[2022-03-02T03:27:08,344][DEBUG][logstash.outputs.elasticsearch][main] Waiting for connectivity to Elasticsearch cluster, retrying in 16s

I’m assuming that I’m using the wrong cert maybe? It’s the cert that is created when the elascticsearch container is spun up, what SHOULD I be using ??