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/frontend/run

@@ -6,10 +6,16 @@ set -e
 # This service is DEVELOPMENT only.
 
 if [ "$DEVELOPMENT" == "true" ]; then
+	. /bin/common.sh
 	cd /app/frontend || exit 1
+	log_info 'Starting frontend ...'
+	HOME=/tmp/npmuserhome
+	export HOME
+	mkdir -p /app/frontend/dist
+	chown -R npmuser:npmuser /app/frontend/dist
 	# If yarn install fails: add --verbose --network-concurrency 1
-	yarn install
-	yarn watch
+	s6-setuidgid npmuser yarn install
+	exec s6-setuidgid npmuser yarn watch
 else
 	exit 0
 fi