How Docker Layer Caching Works: Intermediate Layers Explained for Engineers

When a Docker image is built using the 'docker build' command, Docker executes a four-step cycle for each Dockerfile instruction: creating a temporary container, running the instruction inside it, committing the changes as a new intermediate image layer, and then deleting the temporary container. Each line in a Dockerfile — from the base image pull to file copies — produces a distinct, stackable layer that contributes to the final tagged image. One of Docker's most powerful features is layer caching, where unchanged layers are reused from local cache during subsequent builds, making them near-instantaneous. However, if any instruction in the Dockerfile changes, Docker invalidates the cache for that layer and rebuilds all subsequent layers from scratch. Understanding this internal build lifecycle and caching mechanism is considered essential knowledge for engineers and DevOps professionals aiming to optimize build speed and efficiency.