How does Docker know when to use the cache during a build and when not?

Question

I\'m amazed at how good Docker\'s caching of layers works but I\'m also wondering how it determines whether it may use a cached layer or not.

Let\'s take these build

Answer 1

It's because your package.json file has been modified, see Removing intermediate container.

That's also usually the reason why package-manager (vendor/3rd-party) info files are COPY'ed first during docker build. After that you run the package-manager installation, and then you add the rest of your application, i.e. src.

If you've no changes to your libs, these steps are served from the build cache.