update Node.js version to 10.15.0 (#191)

diff --git a/Dockerfile b/Dockerfile
index 57f8728..d20ba58 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM node:10.14.0
+FROM node:10.15.0
 
 # only package.json
 ADD package.json /