Explain Docker image layers and how they work.

Docker images are built in layers using a union filesystem. Each Dockerfile instruction creates a new layer. How layers work: 1) Each layer is read-only, 2) Layers are stacked on top of each other, 3) Changes create new layers, 4) Layers are shared between images, 5) Only changed layers need to be downloaded. Example: FROM ubuntu:20.04 (Layer 1), RUN apt-get update (Layer 2), RUN apt-get install -y nginx (Layer 3), COPY index.html /var/www/html/ (Layer 4). Benefits: Efficient storage (shared layers), Faster builds (cached layers), Faster downloads (only new layers), Version control for images. Best practices: Combine RUN commands to reduce layers, Order instructions by change frequency, Use .dockerignore to exclude unnecessary files.