Allow to set admin username and email as secret

[keunes]

Hello,

I would like to publish my docker compose files as they are. However, I wouldn't want to publish my LimeSurvey admin username and email address.

Not sure if this request belongs here or if it should go to LimeSurvey directly, but I'd like to be able to also set ADMIN_USER_FILE AND ADMIN_EMAIL_FILE.

Thanks for making Limesurvey available via Docker :-)