server: add Dockerfile and environment variables

The Dockerfile (and docker-compose) simplify the server's deployment.
The environment variables avoid secrets in the code.

server/database.php

@@ -7,20 +7,14 @@ class database
     private static $instance = null;
     private $conn;
 
-    private $host = 'localhost';
-    private $user = 'root';
-    private $pass = '';
-    private $name = 'fe_charts';
-    private $port = '';
-
     // The db connection is established in the private constructor.
     private function __construct()
     {
         try {
             $this->conn = new PDO(
-                "mysql:host={$this->host};dbname={$this->name};port={$this->port}",
-                $this->user,
-                $this->pass
+                'mysql:host=' . getenv('MYSQL_HOST') . ';dbname=' . getenv('MYSQL_DATABASE') . ';port=' . getenv('MYSQL_PORT'),
+                getenv('MYSQL_USER'),
+                getenv('MYSQL_PASSWORD')
             );
         } catch (PDOException $e) {
             die('Connection failed: ' . $e->getMessage());