docker db use native password as auth method

This commit changes the default auth plugin of mysql to
mysql_native_password, because otherwise the laravel app couldn't
connect to the mysql database. The following error was thrown:

`SQLSTATE[HY000] [2054] The server requested authentication method unknown to the client`

docker-compose.yml

@@ -60,6 +60,7 @@ services:
     restart: unless-stopped
     networks:
       - internal
+    command: --default-authentication-plugin=mysql_native_password
     environment:
       - MYSQL_DATABASE=pixelfed
       - MYSQL_USER=${DB_USERNAME}