[Docker]: Connecting PHPMyAdmin to MySQL doesnt work

Instead of starting them one by one, use docker-compose.

Create a docker-compose.yml file

version: '2'
services:
  db:
    image: mysql
    environment:
      MYSQL_ROOT_PASSWORD: my-secret-pw
    ports:
      # just if you also want to access it directly from you host
      # node neede for phpmyadmin
      - "3306:3306"
  phpmyadmin:
    image: phpmyadmin/phpmyadmin
    depends_on:
      - db
    ports:
      - "8080:8080"

Then start it using docker-compose up in the same folder your docker-compose.yml file is located. Access PHPmyadmin using the browser and use 'db' as the hostname of your database, since that is the name of the service in the docker-compose.yml file and therefore can be resolved using dockers internal DNS service to the actual ip of the docker-container. All the links are setup for you automatically.

That's much simpler - docker run overcomplicates things and is not practical for those things - never.

Hint: if docker-compose is not installed on your machine, install it using this official docs https://docs.docker.com/compose/install/ (out of scope)

The configuration file of phpmyadmin/phpmyadmin (/www/config.inc.php) say by default the host name of database server if 'db' :

$hosts = array('db');

So if your database name container is not 'db', you should add the environment variable PMA_HOST= (or PMA_HOSTS if multi db servers) with the right name (databaseContainer in your case)