Don't allow USER_NAME to be set to existing user

Author: thespad

root/etc/s6-overlay/s6-rc.d/init-adduser/run

@@ -1,7 +1,12 @@
 #!/usr/bin/with-contenv bash
 # shellcheck shell=bash
 
-USER_NAME=${USER_NAME:-linuxserver.io}
+if grep -q "^${USER_NAME}" /etc/passwd; then
+    echo "*** USER_NAME cannot be set to an user that already exists in /etc/passwd. Halting init. ***"
+    sleep infinity
+else
+    USER_NAME=${USER_NAME:-linuxserver.io}
+fi
 
 PUID=${PUID:-911}
 PGID=${PGID:-911}