To install packages in a Dockerfile during the build process, use the directive RUN apt-get install, followed by the package names.
The directive in a Dockerfile that allows you to install packages during the image build process is A) RUN apt-get install. This directive is used to execute commands within the container. For example, if you want to install a package like curl, you would include a line in your Dockerfile such as RUN apt-get update && apt-get install -y curl. It's important to run apt-get update before installation commands to ensure that the package indices are updated.