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.sh

@@ -117,8 +117,8 @@ if [ "$CLEAN_BUILD" = true ]; then
     # Remove output artifacts (use sudo if needed for root-owned files)
     if ! rm -rf ../output/* 2>/dev/null; then
         print_info "  Some files owned by root, using container to clean..."
-        export UID=$(id -u)
-        export GID=$(id -g)
+        export USER_UID=$(id -u)
+        export USER_GID=$(id -g)
         docker compose run --rm builder sh -c "rm -rf /build/output/*" || {
             print_warning "Failed to clean output directory, trying with sudo"
             sudo rm -rf ../output/*
@@ -150,8 +150,8 @@ mkdir -p ../output ../cache/github ../cache/packages
 export BUILDMODE
 export MINIMAL_MODE
 export TARGETARCH="${TARGETARCH:-amd64}"
-export UID=$(id -u)
-export GID=$(id -g)
+export USER_UID=$(id -u)
+export USER_GID=$(id -g)
 
 if [ "$DEV_MODE" = true ]; then
     print_info "Starting development container..."