Quite often, one of the most time-taking parts of the build process is the installation of dependencies. This process is traditionally slow because package managers choose stability over performance. And this perfectly makes sense: if something terrible happens, the system must remain in a usable state.
However, stability is not very important when building an image: if the build fails, the system discards the image, and you have to start over.
This post provides some tips on how to use eatmydata to speed up some operations by the example of Debian-based images.