added base image for arch builder

arch-builder/Dockerfile

@@ -1,4 +1,4 @@
-FROM archlinux:latest
+FROM archlinux:latest AS base
 
 COPY mirrorlist /etc/pacman.d/mirrolist
 
@@ -66,6 +66,7 @@ RUN pacman -S --noconfirm --needed \
     clang \
     jdk21-openjdk 
 
+FROM base as rootless
 RUN useradd luser -G users -u 1000 -s /bin/bash -m -d /home/luser
 USER luser
 WORKDIR /home/luser