Add Env to predefine admin user

### Discussed in https://github.com/0xJacky/nginx-ui/discussions/213

<div type='discussions-op-text'>

<sup>Originally posted by **RafaelRodriguezSanz** December  2, 2023</sup>
Hi, I just wanted to know how can I configure an admin user in my dockerfile or docker-compose. I need to skip the install page and directlly go to login with the predefined user. Is it possible?

Thanks!</div>