Let’s say we have the following Dockerfile:

FROM ubuntu:bionic
RUN apt update && apt install git
RUN ["git", "clone", "git@github.com:d4nyll/hobnob.git"]

If we ran this one month ago, and have the layers stored in the cache, and went to build it again today, Docker will use the cached layer, even though the apt sources list is likely to be out of date.

This is done so that you can have a reproducible build. Let’s imagine I made some changes to my code. If I build a new image and it fails, I want to be certain that this is because of the changes I have made, not because of a bug in one of the packages that was silently updated.

If you’d like to disable the cache and build a brand new image, you can do so by passing the --no-cache flag to docker build.