Docker Image Layers

A Docker image consists of a series of read-only layers, each representing an instruction in the image’s Dockerfile. These layers are stacked on top of each other to form the final image.

How Layers Work

• Union File System (UnionFS): Docker uses UnionFS to combine these layers into a single coherent file system.
• Immutability: Each layer is immutable. Once created, it never changes.
• Caching: Docker caches layers. If a Dockerfile instruction hasn’t changed, Docker reuses the existing layer from the cache during the build process, significantly speeding up builds.
• Copy-on-Write (CoW): When a container is started from an image, Docker adds a thin writable layer (the “container layer”) on top of the stack. Any changes made to the running container (like writing new files or modifying existing ones) are written to this thin layer.

Optimization Strategy

To keep images small and builds fast:

1. Order matters: Place instructions that change frequently (like COPY . .) as late as possible in the Dockerfile.
2. Chain commands: Combine multiple RUN commands using && to reduce the number of layers.
3. Clean up: Remove temporary files and package manager caches in the same RUN instruction where they were created.