Update Dockerfile (#9796)

Dockerfile

@@ -1,5 +1,5 @@
-FROM node:8.14.0-alpine as node
-FROM ruby:2.4.5-alpine3.8
+FROM node:8.15-alpine as node
+FROM ruby:2.6-alpine3.8
 
 LABEL maintainer="https://github.com/tootsuite/mastodon" \
       description="Your self-hosted, globally interconnected microblogging community"