Docker reinstalls every dependency when I change one line of code

Question

Every tiny code change means waiting three minutes for npm ci again. My Dockerfile:

FROM node:22-alpine
WORKDIR /app
COPY . .
RUN npm ci
CMD ["node", "server.js"]

Tomasz Wiśniewski · Accepted Answer

Docker caches each step and reuses it until something that step depends on changes. COPY . . depends on every file in your project, so any edit invalidates it, and every step after it reruns, including the install.

Copy the dependency manifests on their own first, install, and only then copy the code:

FROM node:22-alpine
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci
COPY . .
CMD ["node", "server.js"]

Now the install only reruns when the lockfile changes. Also add a .dockerignore containing at least node_modules and .git. Otherwise you copy your local node_modules over the freshly installed one, and the build context is huge too.

Kenji Morita · Answer

And for when the lockfile does change: a BuildKit cache mount keeps npm's download cache between builds, so only new packages get downloaded:

RUN --mount=type=cache,target=/root/.npm npm ci

Leilani Kahale · Answer

Rebuilds after a code change went from three minutes to about four seconds. I would never have guessed the order of two lines mattered that much.