What is the purpose of the `ARG` instruction in a Dockerfile?

The purpose of the ARG instruction in a Dockerfile is to define a variable that users can pass at build-time to the Docker image. When you declare an ARG, you can provide a default value that can be overridden by the user at build time. This flexibility allows for customizing the build process based on specific requirements without hardcoding values directly into the Dockerfile.

For example, using ARG, you can specify different versions of a software package, database connections, or other configuration settings that may vary between environments (such as development and production). To utilize this argument, users can pass it using the --build-arg flag when running docker build, enabling them to customize the image at the time of its creation.

Other instructions mentioned serve different purposes. For instance, the instruction that specifies the base image sets up the foundation upon which the rest of the image is built, while the instruction that sets environment variables configures runtime settings within the container. Lastly, creating a directory inside the container is a straightforward file system operation that doesn't involve user-defined build-time variables. Therefore, ARG distinctly serves to introduce build-time variables, making it essential for parameterizing Docker builds.