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/00-all.sh

@@ -0,0 +1,18 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash
+
+set -e
+
+. /bin/common.sh
+
+if [ "$(id -u)" != "0" ]; then
+	log_fatal "This docker container must be run as root, do not specify a user.\nYou can specify PUID and PGID env vars to run processes as that user and group after initialization."
+fi
+
+. /etc/s6-overlay/s6-rc.d/prepare/10-npmuser.sh
+. /etc/s6-overlay/s6-rc.d/prepare/20-paths.sh
+. /etc/s6-overlay/s6-rc.d/prepare/30-ownership.sh
+. /etc/s6-overlay/s6-rc.d/prepare/40-dynamic.sh
+. /etc/s6-overlay/s6-rc.d/prepare/50-ipv6.sh
+. /etc/s6-overlay/s6-rc.d/prepare/60-secrets.sh
+. /etc/s6-overlay/s6-rc.d/prepare/90-banner.sh