# Docker Layers

**Layers** are stacked filesystem snapshots. Each Dockerfile instruction creates a layer; the image is the union of all layers. Layers are cached—rebuilding reuses unchanged layers, making subsequent builds fast. Inspect layers with `docker history`.

```bash
$ docker history myapp                # show layers and sizes
$ docker inspect myapp:latest         # show layer details in JSON
$ docker build --no-cache .           # rebuild without caching layers
$ docker build -t myapp .             # rebuild (reuses unchanged layers)
```

Smaller layers reduce image size and improve distribution speed. Combine RUN commands with `&&` to reduce layer count: `RUN apt-get update && apt-get install -y curl` creates one layer instead of two. The order of Dockerfile instructions matters for caching—place frequently changing instructions (like `COPY .`) near the end so earlier, stable layers don't rebuild unnecessarily.