fix(docker): base image node:8-alpine and upgrade to npm6 (#567) r=@jbuck,@vladikoff

Dockerfile-build

@@ -1,4 +1,6 @@
-FROM node:8.11.1-alpine
+FROM node:8-alpine
+
+RUN npm install -g npm@6 && rm -rf ~app/.npm /tmp/*
 
 RUN apk add --no-cache git