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/40-dynamic.sh

@@ -0,0 +1,17 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash
+
+set -e
+
+log_info 'Dynamic resolvers ...'
+
+DISABLE_IPV6=$(echo "${DISABLE_IPV6:-}" | tr '[:upper:]' '[:lower:]')
+
+# Dynamically generate resolvers file, if resolver is IPv6, enclose in `[]`
+# thanks @tfmm
+if [ "$DISABLE_IPV6" == "true" ] || [ "$DISABLE_IPV6" == "on" ] || [ "$DISABLE_IPV6" == "1" ] || [ "$DISABLE_IPV6" == "yes" ];
+then
+	echo resolver "$(awk 'BEGIN{ORS=" "} $1=="nameserver" { sub(/%.*$/,"",$2); print ($2 ~ ":")? "["$2"]": $2}' /etc/resolv.conf) ipv6=off valid=10s;" > /etc/nginx/conf.d/include/resolvers.conf
+else
+	echo resolver "$(awk 'BEGIN{ORS=" "} $1=="nameserver" { sub(/%.*$/,"",$2); print ($2 ~ ":")? "["$2"]": $2}' /etc/resolv.conf) valid=10s;" > /etc/nginx/conf.d/include/resolvers.conf
+fi