What are Docker image layers?

Docker images are built from a series of read-only layers, where each layer represents a set of filesystem changes created by a Dockerfile instruction (RUN, COPY, ADD). Layers are stacked on top of each other using a union filesystem (overlay2 is the default storage driver on modern Linux). Each layer stores only the changes (diff) from the previous layer — only files added, modified, or deleted. When you run a container, Docker adds a thin writable layer (container layer) on top — all writes go there; the image layers remain read-only. Layer caching: Docker caches each layer by its instruction + content. When rebuilding, if an instruction and its dependencies haven't changed, Docker reuses the cached layer — making rebuilds fast. Cache is invalidated from the changed instruction downward. Implication for Dockerfile ordering: put instructions that change frequently (COPY source code, ENV) at the end; put instructions that change rarely (installing dependencies) early. Example: COPY package.json first → npm install → COPY rest of source code. This way, npm install is cached if only source changed. Viewing layers: docker image history myapp:1.0 or docker inspect myapp:1.0. Sharing layers: if two images share layers (e.g., same base image), Docker stores the shared layers only once on disk.