Dockerfile: update Node to 8.12.0 and put Ruby on Alpine 3.7 for build system memory help (#8842)

Dockerfile

@@ -1,5 +1,5 @@
-FROM node:8.11.3-alpine as node
-FROM ruby:2.4.4-alpine3.6
+FROM node:8.12.0-alpine as node
+FROM ruby:2.4.4-alpine3.7
 
 LABEL maintainer="https://github.com/tootsuite/mastodon" \
       description="Your self-hosted, globally interconnected microblogging community"