add default avatar upload path

even though default avatar upload is GridFS, many docker users continue to change it to FileSystem

Dockerfile

@@ -30,7 +30,8 @@ WORKDIR /app/bundle
 # needs a mongoinstance - defaults to container linking with alias 'db'
 ENV MONGO_URL=mongodb://db:27017/meteor \
     PORT=3000 \
-    ROOT_URL=http://localhost:3000
+    ROOT_URL=http://localhost:3000 \
+    Accounts_AvatarStorePath=/app/uploads
 
 EXPOSE 3000
 CMD ["node", "main.js"]