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/90-banner.sh

@@ -0,0 +1,17 @@
+#!/command/with-contenv bash
+# shellcheck shell=bash
+
+set -e
+
+echo
+echo "-------------------------------------
+ _   _ ____  __  __
+| \ | |  _ \|  \/  |
+|  \| | |_) | |\/| |
+| |\  |  __/| |  | |
+|_| \_|_|   |_|  |_|
+-------------------------------------
+User UID: $(id -u npmuser)
+User GID: $(id -g npmuser)
+-------------------------------------
+"