Drupal docker-compose mariadb connection error

I'm using Docker4Drupal docker-compose set-up to run a Drupal 7 install from a previous codebase.

This is the docker compose file:

version: "2"

services:
  mariadb:
    image: wodby/mariadb:10.1-2.3.5
    environment:
      MYSQL_ROOT_PASSWORD: password
      MYSQL_DATABASE: drupal
      MYSQL_USER: drupal
      MYSQL_PASSWORD: drupal
    volumes:
      - ./mariadb-init:/docker-entrypoint-initdb.d # Place init .sql file(s) here.
#      - /path/to/mariadb/data/on/host:/var/lib/mysql # I want to manage volumes manually.

  php:
# 2. Images without Drupal – wodby/drupal-php:[PHP_VERSION]-[STABILITY_TAG].
    image: wodby/drupal-php:5.6-3.0.0
    environment:
      PHP_SENDMAIL_PATH: /usr/sbin/sendmail -t -i -S mailhog:1025
      PHP_FPM_CLEAR_ENV: "no"
      DB_HOST: mariadb
      DB_USER: drupal
      DB_PASSWORD: drupal
      DB_NAME: drupal
      DB_DRIVER: mysql
    volumes:
       - docker-sync:/var/www/html # Docker-sync
## For Xdebug profiler files
#      - files:/mnt/files

  nginx:
# wodby/drupal-nginx:[DRUPAL_VERSION]-[NGINX_VERSION]-[STABILITY_TAG].
    image: wodby/drupal-nginx:7-1.13-3.0.1
    depends_on:
      - php
    environment:
      NGINX_STATIC_CONTENT_OPEN_FILE_CACHE: "off"
      NGINX_ERROR_LOG_LEVEL: debug
      NGINX_BACKEND_HOST: php
      NGINX_SERVER_ROOT: /var/www/html
    volumes:
        - docker-sync:/var/www/html # Docker-sync
    labels:
      - 'traefik.backend=nginx'
      - 'traefik.port=80'
      - 'traefik.frontend.rule=Host:drupal.docker.localhost'

  mailhog:
    image: mailhog/mailhog
    labels:
      - 'traefik.backend=mailhog'
      - 'traefik.port=8025'
      - 'traefik.frontend.rule=Host:mailhog.drupal.docker.localhost'

  portainer:
    image: portainer/portainer
    command: --no-auth -H unix:///var/run/docker.sock
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    labels:
      - 'traefik.backend=portainer'
      - 'traefik.port=9000'
      - 'traefik.frontend.rule=Host:portainer.drupal.docker.localhost'

  traefik:
    image: traefik
    command: -c /dev/null --web --docker --logLevel=INFO
    ports:
      - '8000:80'
      - '8080:8080' # Dashboard
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

volumes:
  docker-sync:
    external: true
## For Xdebug profiler
#  files:

Docker-sync is running and the codebase in the container is consistent with the host.

Getting the following error in Drupal:

PDOException: SQLSTATE[HY000] [2002] No such file or directory in lock_may_be_available() (line 167 of /var/www/html/includes/lock.inc).

So I added some debug code to the index file which gives this output:

PDOException Object
(
    [message:protected] => SQLSTATE[HY000] [2002] Connection refused
    [string:Exception:private] => 
    [code:protected] => 2002
    [file:protected] => /var/www/html/index.php
    [line:protected] => 24
    [trace:Exception:private] => Array
        (
            [0] => Array
                (
                    [file] => /var/www/html/index.php
                    [line] => 24
                    [function] => __construct
                    [class] => PDO
                    [type] => ->
                    [args] => Array
                        (
                            [0] => mysql:host=localhost:3306;dbname=drupal
                            [1] => drupal
                            [2] => drupal
                            [3] => Array
                                (
                                    [3] => 2
                                )

                        )

                )

        )

    [previous:Exception:private] => 
    [errorInfo] => 
)

But there's not trouble when I connect on the command line:

docker exec -ti plantation_mariadb_1 mysql -udrupal -p --host=localhost
MariaDB [(none)]>

Also ls /var/log/ is empty in the mariadb container.

Where do I poke next?

UPDATE:

A linked question suggested that the host should be the name of the MySQL container, in the above docker-compose.yml this is mariadb. The following two connections are successful:

# Fill our vars and run on cli
# $ php -f db-connect-test.php
$dbname = 'drupal';
$dbuser = 'drupal';
$dbpass = 'drupal';
$dbhost = 'mariadb';
$link = mysqli_connect($dbhost, $dbuser, $dbpass) or die("Unable to Connect to '$dbhost'");
mysqli_select_db($link, $dbname) or die("Could not open the db '$dbname'");
$test_query = "SHOW TABLES FROM $dbname";
$result = mysqli_query($link, $test_query);
$tblCnt = 0;
while($tbl = mysqli_fetch_array($result)) {
  $tblCnt++;
  #echo $tbl[0]."<br />\n";
}
if (!$tblCnt) {
  echo "There are no tables<br />\n";
} else {
  echo "There are $tblCnt tables<br />\n";
}

try{
    $dbh = new pdo( 'mysql:host=mariadb:3306;dbname=drupal',
                    'drupal',
                    'drupal',
                    array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
    echo "<pre>";
    print_r($dbh);
    echo "</pre>";
    //die(json_encode(array('outcome' => true)));
}
catch(PDOException $ex){
    echo "<pre>";
    print_r($ex);
    echo "</pre>";
    die(json_encode(array('outcome' => false, 'message' => 'Unable to connect')));
}

But the Drupal connection, as far as I can tell, still isn't working.

FURTHER DEBUGGING:

This error occurs in the Drupal bootstrap process before the database credentials are even registered. The function referenced in the error message is in includes/lock.inc and I added print_r(debug_backtrace()) shows the stack:

• lock_may_be_available
• lock_acquire
• variable_initialize
• _drupal_bootstrap_variables
• drupal_bootstrap
• _drupal_bootstrap_page_cache
• drupal_bootstrap

With variable_init being the parameter sent to lock_may_be_available.

I am trying to track down what the SQL string of

db_query('SELECT expire, value FROM {semaphore} WHERE name = :name', array(':name' => $name))->fetchAssoc();

might be so I can run it manually in the MariaDB image.