What is a Dockerfile

A Dockerfile is a text file containing instructions for building a container image. Each instruction is executed in order, and each one that modifies the filesystem creates a new layer.

How it works

Key instructions:

• FROM — sets the base image. Every Dockerfile starts with FROM. FROM ubuntu:22.04 starts from Ubuntu. FROM scratch starts from an empty filesystem.
• RUN — executes a command during the build and saves the result as a layer. RUN apt-get install -y nginx installs nginx into the image.
• COPY — adds files from the build context into the image. COPY app.js /app/ creates a layer with that file.
• CMD — sets the default command when the container starts. Can be overridden at runtime.
• ENTRYPOINT — sets the executable that always runs. CMD provides default arguments to it.
• ENV — sets environment variables.
• EXPOSE — documents which ports the container listens on (does not actually publish them).
• WORKDIR — sets the working directory for subsequent instructions.

docker build processes the Dockerfile, executes each instruction, and produces an image. Layer caching means unchanged instructions reuse cached layers — only changed instructions (and everything after them) are rebuilt.

Why it matters

Dockerfiles are the standard way to define reproducible container builds. They encode the exact steps to go from a base OS to a running application. Multi-stage builds (multiple FROM instructions) separate build dependencies from runtime, producing minimal production images.

See How Container Images Work for layer caching strategies and image optimization.