What Is an Image Layer?

An image layer is a single set of filesystem changes produced by one build step, stored as a content-addressed blob. A container image is an ordered stack of layers that the runtime overlays to produce the final root filesystem. Because layers are addressed by digest, identical layers are stored once and shared across images.

Why it matters

Layer sharing is why pulling a new image often downloads only a few megabytes: unchanged base layers are already cached. Ordering build steps so volatile content sits in late layers maximizes cache reuse and shrinks pushes. On runners, good layering means dependency layers stay cached while only app code changes.

Related guides