add --allow-superuser option to meteor npm install in Dockerfile

It used to work without that option, but it looks like something
in Meteor has changed and that option is now required there.

Dockerfile

@@ -39,7 +39,7 @@ WORKDIR /meteor/meteorapp
 RUN rm -rf .meteor/local
 
 # install node packages
-RUN meteor npm install
+RUN meteor npm install --allow-superuser
 # Expose port 3000 - that is the default port for Meteor applications.
 EXPOSE 3000