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 → Executes command and snapshots the result COPY / ADD → Adds files into a new layer ENV , WORKDIR , CMD → Metadata layers These layers are: Immutable Content-addressed (using SHA256) Reusable across images and builds This design is what makes Docker fast and efficient. 2. How Docker Build Works (Step by Step) When you run docker build . : Docker CLI sends the build context (files + Dockerfile) to the daemon. BuildKit (Docker’s modern build engine) takes control. Dockerfile is read from top to bottom.…