Adds support to run processes as a user/group, defined

with PUID and PGID environment variables

- Detects if image is run with a user in docker command and fails if so
- Adds s6 prepare scripts for adding a 'npmuser'
- Split up and refactor the s6 prepare scripts
- Runs nginx and backend node as 'npmuser'
- Changes ownership of files required at startup

README.md

@@ -70,6 +70,8 @@ services:
       - ./letsencrypt:/etc/letsencrypt
 ```
 
+This is the bare minimum configuration required. See the [documentation](https://nginxproxymanager.com/setup/) for more.
+
 3. Bring up your stack by running
 
 ```bash