In a Dockerfile, what does the 'FROM' instruction do?

The 'FROM' instruction in a Dockerfile is crucial as it specifies the base image upon which the new Docker image will be built. This instruction essentially tells Docker which pre-existing image to use as a starting point. For instance, if you're creating a web application, you might choose a base image like Ubuntu, Alpine, or a specific language image like Python or Node.js, which already contains the necessary environment and libraries.

By defining a base image, the 'FROM' instruction also sets the stage for subsequent instructions in the Dockerfile, allowing you to layer custom configurations, dependencies, and instructions on top of that base. This layering system is one of Docker's powerful features, enabling image reuse and minimizing redundancy.

In summary, the 'FROM' instruction provides the foundational platform for building your application inside a container, making it a fundamental aspect of Docker image creation.