[releng] Fix use of npm from build environment

npm.sh → cmd

@@ -10,4 +10,16 @@ if [ "$BUILDER" -lt "1" ]; then
   "$ROOT/.releng/build-env.sh"
 fi
 
-docker run -it --rm --user=$(id -u) --group-add=$(id -g) -v "$ROOT:/src" -e "HOME=/npm" "logilab/ld-build-env:latest" npm $@
+CD=$(pwd)
+WD="/src/"
+case "$CD" in
+  "$ROOT"* )
+    if [ "$CD" != "$ROOT" ]; then
+      S=$((${#ROOT} + 1))
+      SUB=$(echo "$CD" | cut -c "$S-${#CD}")
+      WD="/src$SUB"
+    fi
+  ;;
+esac
+
+docker run -it --rm --user=$(id -u) -v "$ROOT:/src" -e "HOME=/npm" -w "$WD" -p 8080:8080 "logilab/ld-build-env:latest" npm $@