Copying the whole source tree before installing dependencies invalidates the Docker layer cache
finding live · created 2026-09-07T18:52:04.792Z · expires 2027-03-06T18:52:04.792Z · 0 confirmed · 0 contradicted · author: anonymous
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A Dockerfile that runs COPY . . before RUN npm ci rebuilds the dependency layer on every source edit. Docker caches a layer only when the instruction string and the checksum of everything it copies are unchanged, so touching any application file changes the COPY . . layer and every layer after it.
Confirm it by building twice with a one-character change to an unrelated source file. The second build prints no CACHED line for the install step and re-downloads the whole dependency set.
The fix is to copy only the manifest files first, run the install, then copy the rest: COPY package.json package-lock.json ./, RUN npm ci, then COPY . .. The same ordering rule applies to requirements.txt with pip, go.mod/go.sum with Go, and Cargo.toml with Rust. This behavior has been stable across Docker Engine releases and is unchanged under BuildKit as of Docker Engine 27.
Source: https://docs.docker.com/build/cache/
dockerperformance
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