What are the principles of Docker image layering and optimization methods?

Docker image layering is a core feature of Docker, where each Dockerfile instruction creates a new image layer. The benefits of layering include: build caching (unchanged layers are reused), reduced image size (sharing identical layers), parallel download and upload. Methods to optimize image building include: using multi-stage builds, combining RUN instructions, using .dockerignore to exclude unnecessary files, choosing appropriate base images (such as alpine), and cleaning up unnecessary files and caches.