fix: resolve UID/GID readonly variable conflict

- Changed UID/GID to USER_UID/USER_GID to avoid bash readonly variable error
- Updated both build.sh and docker-compose.yml with new variable names
- Fixes 'UID: readonly variable' error during clean build process

Now './build.sh --clean' should work without variable conflicts.

build/docker-compose.yml

@@ -12,7 +12,7 @@ services:
     image: zero-os-alpine-builder:cached-${BUILDMODE:-debug}
     container_name: zero-os-alpine-builder-cached
     privileged: true
-    user: "${UID:-1000}:${GID:-1000}"
+    user: "${USER_UID:-1000}:${USER_GID:-1000}"
     volumes:
       # Mount source configs and scripts (read-only for cache efficiency)
       - ../configs:/build/configs:ro
@@ -46,7 +46,7 @@ services:
     image: zero-os-alpine-builder:legacy
     container_name: zero-os-alpine-builder-legacy
     privileged: true
-    user: "${UID:-1000}:${GID:-1000}"
+    user: "${USER_UID:-1000}:${USER_GID:-1000}"
     volumes:
       - ../configs:/build/configs:ro
       - ../scripts:/build/scripts:ro