A Dockerfile is a text file containing step-by-step instructions that define how to build a container image. Each instruction—such as copying files, installing dependencies, or setting environment variables—creates a new layer in the final image. Dockerfiles provide a reproducible way to package an application and all its dependencies so that it runs consistently across environments. They are processed by the Docker build engine, which assembles the image in the order of the listed commands.
Why it matters
Dockerfiles ensure consistent, automated builds of containerized applications. Without them, teams would rely on ad-hoc manual setup steps that are difficult to maintain and error-prone. Dockerfiles integrate seamlessly with CI/CD pipelines, cloud platforms, and orchestration tools like Kubernetes.
Examples
Using FROM node:18-alpine to define a base image or COPY . /app to include project files are common Dockerfile commands. Lessons like Writing Dockerfiles walk through these instructions.