What is the role of a Dockerfile's FROM instruction?

The role of a Dockerfile's FROM instruction is to designate the base image for the Docker image being built. When creating a new Docker image, it is essential to specify an existing image that serves as the foundation. This base image can be a fully-functional operating system image (like Ubuntu or Alpine) or it could be an application-specific image (such as one for Node.js or Python). By using the FROM instruction, you set the starting point for your custom image, allowing you to build on top of the functionalities and layers provided by the chosen base image.

This foundational aspect is critical in the Docker image build process, as it determines the software environment, libraries, and tools available in your container. The capabilities and compatibility of the final image are influenced heavily by the base image selected. Once a base image is specified, subsequent instructions in the Dockerfile can add to this image by installing additional software, copying files, and configuring the environment further.