Dockerfile & Image Build Internals: From Layers to Lightning-Fast Builds

You write a Dockerfile , run docker build , and get an image. But what’s really happening under the hood? Docker isn’t just “building” your app — it’s assembling a stack of immutable filesystem layers . Docker doesn’t build applications — it builds filesystem snapshots layer by layer . Let’s break it down. 1. What is a Docker Image, Really? A Docker image is not a single file . It’s a stack of read-only layers . Every instruction in your Dockerfile creates a new layer: FROM → Base layer RUN → Ex