Undo mistake in Dockerfile

Dockerfile

@@ -1,11 +1,12 @@
 FROM node:lts-alpine
 WORKDIR /usr/src/fosscord-gateway
-COPY . .
+COPY package.json .
 RUN apk --no-cache --virtual build-dependencies add \
     python \
     make \
     g++
 RUN npm install
 RUN apk del build-dependencies
+COPY . .
 EXPOSE 3002
 CMD ["npm", "start"]