What does the 'COPY' instruction do in a Dockerfile?

The 'COPY' instruction in a Dockerfile is utilized to transfer files or directories from the host (the machine where you run the Docker daemon) to the image being built. This is essential for including application code, configuration files, and any resources that your application might need at runtime in the container.

When using the 'COPY' instruction, you specify the source path on the host and the destination path in the image. This effectively embeds those files into the image, allowing the container created from this image to have access to them when it runs. The 'COPY' command is a fundamental component of creating Docker images since it facilitates the packaging of all necessary components together into one deployable unit.

In contrast, the other options do not accurately describe the functionality of the 'COPY' instruction. For example, copying the entire image or creating a backup is not related to the 'COPY' command's purpose, nor does it deal with metadata, which is managed separately during the image build process. Hence, the clear definition of 'COPY' focusing on transferring files from the host to the image makes option B the right choice.