bump node minor version from 10.19.0 to 10.20.1 (#225)

diff --git a/Dockerfile b/Dockerfile
index 8d26e71..0b34a41 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,7 +15,7 @@
 # limitations under the License.
 #
 
-FROM node:10.19.0
+FROM node:10.20.1
 
 # only package.json
 ADD package.json /