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/prepare/30-ownership.sh

@@ -0,0 +1,21 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash
+
+set -e
+
+log_info 'Setting ownership ...'
+
+# root
+chown root /tmp/nginx
+
+# npmuser
+chown -R npmuser:npmuser \
+	/data \
+	/etc/letsencrypt \
+	/etc/nginx \
+	/run/nginx \
+	/tmp/nginx \
+	/var/cache/nginx \
+	/var/lib/logrotate \
+	/var/lib/nginx \
+	/var/log/nginx