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/docker-compose.dev.yml

@@ -14,6 +14,8 @@ services:
     networks:
       - nginx_proxy_manager
     environment:
+      PUID: 1000
+      PGID: 1000
       NODE_ENV: "development"
       FORCE_COLOR: 1
       DEVELOPMENT: "true"