upgrade Node.js to pick up security fixes (#177)

diff --git a/Dockerfile b/Dockerfile
index f430f4e..26d126d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM node:10.15.0
+FROM node:10.15.2
 
 # only package.json
 ADD package.json /