Warn processing nginx.conf with a custon user PUID

Checklist

• Have you pulled and found the error with jc21/nginx-proxy-manager:latest docker image?
  • Yes
• Are you sure you're not using someone else's docker image?
  • Yes
• Have you searched for similar issues (both open and closed)?
  • Yes

Describe the bug
When running npm with a custom user:

enviroment:
  - PUID=1000
  - PGID=1000

Warn occur when processing the nginx.conf file, and this warn is logged at fallback_error.log:

2023/10/02 19:47:29 [warn] 158#158: the "user" directive makes sense only if the master process runs with super-user privileges, ignored in /etc/nginx/nginx.conf:4

Nginx Proxy Manager Version
v2.10.4

To Reproduce
Steps to reproduce the behavior:

1. Configure docker-compose.yml whit a custom user

enviroment:
  - PUID=1000
  - PGID=1000

2. Start the container
3. check ./data/logs/fallback_access.log

Expected behavior
I think the proper behavior should be automatically change nginx.conf file: user npm to user PUID, when PUID is configured as an environment variable.

Perhaps is possible set a default value in the environment variable to npm, and allways set this value in nginx.conf when the container starts.

Operating System

Linux pi4 6.1.21-v8+ #1642 SMP PREEMPT Mon Apr  3 17:24:16 BST 2023 aarch64 GNU/Linux

Start with the docker-compose.yml environment settings and the generated /etc/nginx/nginx.conf, then reproduce the warning using custom PUID and PGID values. Check the container startup path that processes nginx.conf and verify completion by confirming the fallback log no longer reports the ignored user directive while the container still starts correctly.