Upgrade Node.js to 12 since it's the new LTS (#62)

Author: gengjiawen

Dockerfile

@@ -8,7 +8,7 @@ ARG ANDROID_BUILD_VERSION=29
 ARG ANDROID_TOOLS_VERSION=29.0.2
 ARG BUCK_VERSION=2019.10.17.01
 ARG NDK_VERSION=20
-ARG NODE_VERSION=10.x
+ARG NODE_VERSION=12.x
 ARG WATCHMAN_VERSION=4.9.0
 
 # set default environment variables