Image Layers
A Docker image isn't one monolithic file — it's read-only layers stacked on top of each other. Each Dockerfile instruction creates one layer, and at run time a writable container layer is added on top so the whole stack looks like a single filesystem. Identical layers are shared between images and cached, which makes instruction order the thing that decides your build speed and image size.
01 Concept at a Glance
Interactive Step-by-StepA Docker image isn't one monolithic file. It's read-only layers stacked on top of each other, and each Dockerfile instruction creates one layer.
02 Understand It Simply
For EveryoneLike finishing a drawing on stacked transparency sheets. The bottom sheet (the base image) stays put and you only add new sheets on top; look through them and you see one picture (the filesystem). Swap a sheet in the middle and every sheet above it has to be redrawn.
Dockerfile instructions like FROM, COPY, and RUN each create a read-only layer, and a union filesystem overlays them into one view.
Start a container and a writable layer is added on top — every change is recorded only there (copy-on-write).
Changing a layer invalidates the cache of every layer above it, so the key is to put what rarely changes (installing dependencies) low and what changes constantly (your source) high.
- –Ordering Dockerfile instructions
- –raising build cache hit rates
- –shrinking image size
- –and understanding how multiple images share base layers
03 Frequently Asked Questions
FAQWhat is Image Layers?+
A Docker image isn't one monolithic file — it's read-only layers stacked on top of each other. Each Dockerfile instruction creates one layer, and at run time a writable container layer is added on top so the whole stack looks like a single filesystem. Identical layers are shared between images and cached, which makes instruction order the thing that decides your build speed and image size.
Where is Image Layers used?+
Ordering Dockerfile instructions, raising build cache hit rates, shrinking image size, and understanding how multiple images share base layers.
What's a simple analogy for Image Layers?+
Like finishing a drawing on stacked transparency sheets. The bottom sheet (the base image) stays put and you only add new sheets on top; look through them and you see one picture (the filesystem). Swap a sheet in the middle and every sheet above it has to be redrawn.