Bump node build version

When trying to build the Docker image, I kept receiving an out-of-memory
error from Webpack. This issue is apparently explained at
webpack/webpack#1914.
Several comments in the issue suggested using Node 12 or above. Doing so
resolved this issue for me.

Signed-off-by: Matt Cengia <mattcen@mattcen.com>

Dockerfile

@@ -1,5 +1,5 @@
 # Builder
-FROM node:10 as builder
+FROM node:12 as builder
 
 # Support custom branches of the react-sdk and js-sdk. This also helps us build
 # images of riot-web develop.