chore(Dockerfile): install with --production, rejigger layers (#23)

Dockerfile

@@ -1,18 +1,15 @@
 FROM node:10.15.1-alpine
 
-# Set ENV vars
 ENV NODE_ENV production
 ENV NPM_CONFIG_LOGLEVEL info
 
+RUN npm install npm@6.4.1 -g
+
 # Setup source directory
 RUN mkdir /app
 WORKDIR /app
 COPY package.json package-lock.json /app/
-
-# npm install
-ARG skip_install=
-RUN npm install npm@6.4.1 -g
-RUN if [ -z "$skip_install" ]; then npm ci; fi
+RUN npm ci --production
 
 # Copy app to source directory
 COPY . /app