Merge pull request #1118 from linuxserver/universal-docker-in-docker-alpine

universal-docker-in-docker: update alpine, fix var check

Author: aptalca

Dockerfile

@@ -1,13 +1,13 @@
 # syntax=docker/dockerfile:1
 
 ## Buildstage ##
-FROM ghcr.io/linuxserver/baseimage-alpine:3.22 AS buildstage
+FROM ghcr.io/linuxserver/baseimage-alpine:3.23 AS buildstage
 
 ARG MOD_VERSION
 
 RUN \
   echo "**** retrieve latest version ****" && \
-  if [ -z "${MOD_VERSION+x}" ]; then \
+  if [ -z "${MOD_VERSION}" ]; then \
     DOCKER_RELEASE=$(curl -sX GET "https://api.github.com/repos/moby/moby/releases/latest" \
       | awk '/tag_name/{print $4;exit}' FS='[""]' \
       | sed 's|^docker-||' \