Testing search platform. fail

I set up NC 16.0.3 and Elasticsearch 7.2.0/7.3.0 with my playbook:
https://github.com/ReinerNippes/nextcloud_on_docker/tree/elasticsearch

Elasticsearch is running and reachable from the NC container (curl).
Nevertheless the platform tests fail and no index is create.

ec2-user@ip-172-31-7-157:~/nextcloud-docker-git$ docker exec -u root nextcloud curl http://elasticsearch_tesseract:9200/?pretty
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0{
  "name" : "149d5d095127",
  "cluster_name" : "docker-cluster",
  "cluster_uuid" : "QphMvW-8QuehxD0AzFaaqA",
  "version" : {
    "number" : "7.3.0",
    "build_flavor" : "default",
    "build_type" : "docker",
    "build_hash" : "de777fa",
    "build_date" : "2019-07-24T18:30:11.767338Z",
    "build_snapshot" : false,
    "lucene_version" : "8.1.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
  },
  "tagline" : "You Know, for Search"
}
100   508  100   508    0     0   4096      0 --:--:-- --:--:-- --:--:--  4130
ec2-user@ip-172-31-7-157:~/nextcloud-docker-git$ docker exec -u www-data nextcloud php occ fulltextsearch:test

.Testing your current setup:
Creating mocked content provider. ok
Testing mocked provider: get indexable documents. (2 items) ok
Loading search platform. (Elasticsearch) ok
Testing search platform. fail
In Test.php line 301:

  Search platform (Elasticsearch) down ?


fulltextsearch:test [--output [OUTPUT]] [-j|--json] [-d|--platform_delay PLATFORM_DELAY]

ec2-user@ip-172-31-7-157:~/nextcloud-docker-git$ docker exec -u www-data nextcloud php occ fulltextsearch:index

In Index.php line 381:

  failed platform test.

fulltextsearch:index [--output [OUTPUT]] [-r|--no-readline] [--] [<options>]

Start with the fulltextsearch:test command and the failure reported at Test.php line 301, then trace the platform check before reviewing Index.php line 381. Reproduce the failure with the reported Nextcloud and Elasticsearch versions; done means the platform test passes and fulltextsearch:index can create an index.