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/10-npmuser.sh

@@ -0,0 +1,18 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash
+
+set -e
+
+PUID=${PUID:-911}
+PGID=${PGID:-911}
+
+# Add npmuser user
+log_info 'Creating npmuser ...'
+
+groupmod -g 1000 users || exit 1
+useradd -u "${PUID}" -U -d /data -s /bin/false npmuser || exit 1
+usermod -G users npmuser || exit 1
+groupmod -o -g "$PGID" npmuser || exit 1
+# Home for npmuser
+mkdir -p /tmp/npmuserhome
+chown -R npmuser:npmuser /tmp/npmuserhome