improved caching in docker build

Dockerfile

@@ -2,12 +2,11 @@ FROM node:12.13.0-alpine
 
 RUN apk add --no-cache git
 
-COPY . /app
+COPY package.json package-lock.json /app/
-
 WORKDIR /app
-
 RUN rm package-lock.json
-
 RUN npm install
 
+COPY . /app
+
 CMD ["npm", "start"]