Add more information to the docket hub for quick start

[kovalroma]

When I first run this docker-compose

version: '3'

services:
  elkarbackup:
    image: elkarbackup/elkarbackup:1.3.1-apache
    environment:
      SYMFONY__DATABASE__PASSWORD: "your-password-here"
      EB_CRON: "enabled"
    volumes:
      - backups:/app/backups
      - uploads:/app/uploads
      - sshkeys:/app/.ssh
    ports:
      - 8000:80

  db:
    image: mysql:5.7.22
    environment:
      MYSQL_ROOT_PASSWORD: "your-password-here"
    volumes:
      - db:/var/lib/mysql

  client:
    image: elkarbackup/client

volumes:
  db:
  backups:
  uploads:
  sshkeys:

I thought that SYMFONY__DATABASE__PASSWORD: and MYSQL_ROOT_PASSWORD: should to be different. It was not clear to me that elkarbackup use password from root MySQL user to create database. After I've inspected logs I found it. So maybe it's good idea to add more description like MYSQL_ROOT_PASSWORD and SYMFONY__DATABASE__PASSWORD should to be the same.

In additional I think we need to add that default username and password for new application is root/root. I found it in documentation but it's not clear if you read only description in docker hub