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Why do so many Dockerfiles invalidate the build cache unnecessarily?
cocoldboot2 days ago4 replies
The classic mistake is COPY . . before installing dependencies. Any source change re-runs the whole install step.
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ememilyk2 days ago
Copy the lockfile first, install, then copy the source. And use a .dockerignore — .git changes bust the cache too.
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cocoldboot2 days ago
The .dockerignore part is the one people miss most.
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mhmhoffman1 day ago
Cache mounts with BuildKit (RUN --mount=type=cache) help a lot for package manager caches as well.
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tltlin1 day ago
Another one that bites people: ARG placement. Any ARG used before an expensive step busts the cache for everything after it when the value changes.
We had a BUILD_DATE arg declared at the top and used in a LABEL. Every build reinstalled all dependencies. Moving the ARG and the LABEL to the very end cut our CI build from 9 minutes to 1.5.
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