Ordering a Dockerfile So the Cache Survives

The fix is to put the thing that rarely changes above the thing that changes constantly.

Reordering so a source edit stops reinstalling dependencies

Step 1 of 4

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COPY package.json package-lock.json ./

These two files change when a dependency changes, and not when you edit a function.

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RUN npm ci

This layer now depends on the manifests alone, so it survives every edit that does not touch them.

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COPY . .

The layer that changes most often is now the last one, so what it invalidates is only itself and what follows.

bash

$ time docker build -t my-api:1 .   # after a one-line source edit
real	0m1.9s

Against roughly twelve seconds for the same edit in the other order.

The rule generalises past Node: copy the dependency manifest, install, then copy the source. Every ecosystem has the same two files.