Add support for IDM_ADMIN_PASSWORD_FILE #2982

Is your feature request related to a problem? Please describe.

This Issue is related to this one opencloud-compose/#306

In short, the request was to add the variable IDM_ADMIN_PASSWORD_FILE in addition to the variable IDM_ADMIN_PASSWORD, so that, for example, Docker secrets could be used. Currently, opencloud-compose can pass IDM_ADMIN_PASSWORD, but file-based secret handling would require a shell wrapper or entrypoint workaround in the compose stack.

Describe the solution you'd like

Add native server-side support for reading the admin password from a file.

Suggested behavior:

• Support IDM_ADMIN_PASSWORD_FILE
• Optionally also support ADMIN_PASSWORD_FILE, matching the existing ADMIN_PASSWORD alias
• If *_FILE is set, read the password from that file
• Trim a trailing newline from the file content
• If both IDM_ADMIN_PASSWORD and IDM_ADMIN_PASSWORD_FILE are set, return a clear configuration error instead of silently choosing one

This would allow compose deployments to use Docker secrets like:

environment:
  IDM_ADMIN_PASSWORD_FILE: /run/secrets/idm_admin_password
secrets:
  - idm_admin_password

Describe alternatives you've considered

The alternative would be adding a shell wrapper in opencloud-compose that reads the secret file and exports IDM_ADMIN_PASSWORD before starting the server.

I think this is less ideal because:

• it only helps the compose stack
• it duplicates secret-handling logic outside the server
• it is harder to test and maintain
• other deployment methods would not benefit from it

Native server-side support would work for Docker Compose, Kubernetes, systemd, and any other deployment method that can mount secrets as files.

Additional context