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

docker/rootfs/etc/nginx/nginx.conf

@@ -1,7 +1,6 @@
 # run nginx in foreground
 daemon off;
-
-user root;
+pid /run/nginx/nginx.pid;
 
 # Set number of worker processes automatically based on number of CPU cores.
 worker_processes auto;
@@ -57,7 +56,7 @@ http {
 	}
 
 	# Real IP Determination
-	
+
 	# Local subnets:
 	set_real_ip_from 10.0.0.0/8;
 	set_real_ip_from 172.16.0.0/12; # Includes Docker subnet