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/s6-overlay/s6-rc.d/nginx/run

@@ -3,5 +3,8 @@
 
 set -e
 
-echo "❯ Starting nginx ..."
-exec nginx
+. /bin/common.sh
+
+log_info 'Starting nginx ...'
+
+exec s6-setuidgid npmuser nginx