What is a Dockerfile?

A Dockerfile is indeed a text file that contains specific instructions for building a Docker image. This file allows developers to define the environment and dependencies required for their application, specifying various commands that the Docker engine will execute to assemble the image step by step.

The Dockerfile typically includes directives such as FROM, which sets the base image; RUN, which executes commands during the image build process; COPY, which adds files from the host into the image; and CMD, which indicates the default command to run when a container is instantiated from the image. By using a Dockerfile, developers can automate the image building process, ensuring consistency and reproducibility across different environments.

The other options do not capture the fundamental purpose of a Dockerfile. While a user manual could be informative, it does not define how to construct an image. A command-line interface refers to the tools used to interact with Docker rather than the image-building instructions themselves. Lastly, although a configuration file might imply settings for container management, it does not specifically encompass the detailed instructions necessary for creating a Docker image. Thus, the definition of a Dockerfile as a text file with instructions aligns perfectly with its function and purpose in the Docker ecosystem.