Allow setting admin user email via environment variable during initial setup

After a fresh Docker-based install with all SMTP settings supplied via environment variables, the send test via "Basic settings → Test and verify email settings" result in the following error:
AxiosError: Request failed with status code 400
This error message is not helpful and gives no indication of the root cause. It took me several days of debugging - including repeatedly verifying and adjusting the SMTP settings - before I realized that the problem was simply that the admin user had no email address set.

Expected Behavior

A fresh installation should allow specifying an admin email during setup. Without it, the email system silently fails with unclear error messages, despite all SMTP settings appearing correct.

Proposed Solution

Introduce a new environment variable, such as NEXTCLOUD_ADMIN_EMAIL, which can be used during container initialization to automatically assign an email address to the admin user.

environment:
  - NEXTCLOUD_ADMIN_USER=admin
  - NEXTCLOUD_ADMIN_PASSWORD=secret
  - NEXTCLOUD_ADMIN_EMAIL=admin@example.com

Benefits

• Prevents misleading errors like AxiosError: Request failed with status code 400 when testing email
• Reduces time wasted on debugging SMTP configs
• Streamlined, fully automated deployment
• Enables functional email notifications from the start
• Avoids manual post-installation steps

This would align well with the goals of containerization and automation. Thanks for considering!

Start by tracing the container initialization path that handles NEXTCLOUD_ADMIN_USER and NEXTCLOUD_ADMIN_PASSWORD, then determine where a proposed admin email variable would be applied. Reproduce a fresh Docker-based install with SMTP settings and use “Basic settings → Test and verify email settings” to confirm that the email is set and the test no longer fails with the reported 400 error.