Support setting important system configuration values during Docker initialization

[marscat5]

When deploying Nextcloud using the official Docker image, certain important system configuration values are not set by default, resulting in warnings or suboptimal behavior even after a clean installation. Two common examples are:

1. maintenance_window_start
   Fresh installations display this warning in the system overview: "Server has no maintenance window start time configured."
   There is currently no way to set this value during the Docker-based setup. Instead, it must be configured manually via OCC: ./occ config:system:set maintenance_window_start --type=integer --value=1
2. default_phone_region
   Fresh installations display in the system overview: "Your installation has no default phone region set"
   There is currently no way to set this value during the Docker-based setup. Instead, it must be configured manually via OCC: ./occ config:system:set default_phone_region --value="DE"

Expected Behavior

A fresh Nextcloud Docker installation should result in a fully configured, clean state, without requiring manual OCC calls for essential system settings.

Proposed Solution A

Introduce environment variables that allow setting all values at container startup which can result in warnings or messages on the "Security & setup warnings" page.

environment:
  - NEXTCLOUD_DEFAULT_PHONE_REGION=DE
  - NEXTCLOUD_MAINTENANCE_WINDOW_START=1

Proposed Solution B

Offer a documented and supported way to inject occ commands during first-time initialization.

Impact

• Reduces post-installation steps
• Improves automation and reproducibility
• Avoids confusing warnings for new users
• Aligns with Docker/container best practices

Thanks for your time and for maintaining the Docker image!