improve Docker layer caching

Dockerfile

@@ -1,18 +1,22 @@
-FROM node:alpine
-
+FROM node:alpine as builder
 WORKDIR /app
-ADD . /app
 
-RUN apk add --update git bash && \
-	npm install --production && \
-	apk del git && \
-	rm -rf /tmp/* /var/cache/apk/*
+RUN apk add --update git bash
+
+ADD package.json /app
+RUN npm install --production
+
+FROM node:alpine
+WORKDIR /app
 
 EXPOSE 3000
 
 ENV HOSTNAME v5.db.transport.rest
 ENV PORT 3000
 
+COPY --from=builder /app/node_modules ./node_modules
+ADD . /app
+
 RUN npm run build
 
 CMD ["node", "index.js"]