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Correct (as far as I know), but:

A) Changing a Dockerfile is rare B) Typically the lines that change (adding your code) are near the end of the Dockerfile, and the long part with installing libraries is at the beginning

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(A) is true, but it's not the only way to bust the cache. Changes to the filesystem are much more common. You can try to account for this by selectively copying in files in some sort of dependency order and this can work okay so long as the dependency order resembles your filesystem hierarchy, but if you want to (for example) install all of your third party dependencies before adding your source code, you'll need to add each requirements.txt or package.json file individually so as to avoid copying in your source code (which changes more frequently). Doing this also tightly-couples your Dockerfile to your in-tree dependency structure, and keeping the Dockerfile in sync with your dependency tree is an exercise in vanity. Further, because you're forcing a tree-structure (dependency tree) into a linear structure, you are going to be rebuilding a bunch of stuff that doesn't need to be built (this gets worse the wider your dependency tree). Maybe you can hack around this by making an independent build stage per in-tree package, which might allow you to model your dependency tree in your Dockerfile, but you're still left keeping this in sync manually. No good options.

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