10 Make builds fast with the layer cache

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Goal

Reorder the Dockerfile so package.json is copied and npm install runs before the rest of the code is copied, then rebuild myapp.

Docker caches every layer. On a rebuild it reuses a layer if the instruction is the same and everything before it was reused — for COPY, that includes the files being identical. The first step that changes busts the cache for every step after it.

This Dockerfile copies all the code, then runs npm install. So any edit to any file — one line in server.js — invalidates the COPY, and npm reinstalls every dependency. Slow.

Copy package.json alone first, install, then copy the rest. Now code edits only rebuild the last cheap layer; dependencies come from cache until package.json itself changes.

Try it

Fix the order and rebuild. Then edit server.js (echo '// tweak' >> server.js) and rebuild again — watch RUN npm install say CACHED.

Go to the terminal

Hint

In nano Dockerfile, put COPY package.json ./ and RUN npm install before COPY . .. Then docker build -t myapp ..

Show a solution

nano Dockerfile
# In the editor: FROM node:20-alpine · WORKDIR /app · COPY package.json ./ · RUN npm install · COPY . . · EXPOSE 3000 · CMD ["node", "server.js"]
docker build -t myapp .

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Docker

Starting Docker…