Update Dockerfile to alpine:3.9 (why didn't I do this before?)

docker/Dockerfile

@@ -1,11 +1,12 @@
-FROM alpine:3.8
+FROM alpine:3.9
 
 RUN apk update && apk upgrade
 RUN apk add s6
 
 # NB: From Alpine 3.9 onwards, the versions of nodejs and npm in the
 # standard packageset are fine for Syndicate
-RUN apk add --no-cache nodejs npm --repository http://dl-cdn.alpinelinux.org/alpine/edge/main
+# RUN apk add --no-cache nodejs npm --repository http://dl-cdn.alpinelinux.org/alpine/edge/main
+RUN apk add --no-cache nodejs npm
 
 RUN npm i -g preserves
 ENV NODE_PATH=/usr/lib/node_modules