fix(docker): base image node:8-alpine and upgrade to npm6 mozilla/fxa-basket-proxy#59) r=@vladikoff

packages/fxa-basket-proxy/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 addgroup -g 10001 app && \
     adduser -D -G app -h /app -u 10001 app