Platform & DevOps·Packaging: containers and images
changing one line at the top of the build file invalidates every layer beneath it and the build goes from nine seconds to four minutes.
Image layer
Also calledlayer, filesystem layer
One filesystem diff produced by a single build step, content-addressed and stacked with the others to form the image. Layers are shared and cached across images, which is what makes pulling a new build of the same service cheap. The whole practical skill of writing a build file is ordering steps so the slow, rarely-changing ones sit underneath the fast, always-changing ones.