Fixed Dockerfile.gui-extension-builder

Change-Id: Ib3221beab14cdd1f993c8ee925beb5e69c10dc22
diff --git a/Dockerfile.xos-gui-extension-builder b/Dockerfile.xos-gui-extension-builder
index 4bf1e02..523e69c 100644
--- a/Dockerfile.xos-gui-extension-builder
+++ b/Dockerfile.xos-gui-extension-builder
@@ -6,6 +6,8 @@
 ENV CODE_SOURCE .
 ENV CODE_DEST /var/www
 
+RUN mkdir -p /var/www
+
 # Add the app deps
 COPY ${CODE_SOURCE}/package.json ${CODE_DEST}/package.json
 COPY ${CODE_SOURCE}/typings.json ${CODE_DEST}/typings.json