# Docker Dockerfile

**Dockerfile** is a text file listing instructions to build an image. Start with a base image (e.g., `FROM ubuntu:22.04`), then layer changes on top with `RUN`, `COPY`, `WORKDIR`, `ENV`, etc. Docker builds the image by executing each instruction sequentially, creating a new layer for each step.

```dockerfile
FROM ubuntu:22.04

RUN apt-get update && apt-get install -y gcc make

COPY . /app
WORKDIR /app

RUN make

CMD ["./myprogram"]
```

Each instruction creates a layer. Docker caches layers—rebuilding reuses unchanged layers up to the first change, then re-executes from there. Put instructions that change frequently (like `COPY .`) near the end to maximize cache hits. Never split `apt-get update` and `apt-get install` into separate `RUN` instructions—the install might use a stale cached update layer.