Cache never hits in CI even though source code didn't change

Every CI build re-runs `npm install` from scratch even when package.json is identical to the last commit. What would you check? Expected reasoning: confirm COPY package.json comes before COPY . . in the Dockerfile; check whether the CI uses --no-cache or a fresh build context each time (e.g., shallow clone changing file timestamps/hashes unexpectedly, or CI not persisting the Docker build cache between jobs — may need BuildKit's --cache-from/--cache-to or registry cache export).