Which Steps Rebuild?

Given this Dockerfile, work out what each change costs before you look.

Dockerfiletext

FROM node:20-alpine
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci
COPY . .
CMD ["node", "server.js"]

You edit a comment in server.js. Which instructions run again?

COPY . . and CMD. The copy's layer changed, so everything after it reruns — but npm ci is above the copy and keeps its cache. CMD sets metadata and costs nothing.

You add a dependency to package.json. Which instructions run again?

COPY package.json package-lock.json ./ onwards, so the install as well. That is correct: the dependencies really did change.

You change FROM node:20-alpine to FROM node:22-alpine. Which instructions run again?

All of them. The first layer changed, so nothing below it has a cached layer to sit on.

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