Add Build.mk variable for listing files to be copied into the workdir volume

[C0deH4cker]

Currently, a flag.txt file will be automatically detected and copied into the workdir volume. Also, if a project directory contains a workdir/ folder, its contents will be automatically copied into the workdir volume. It would be nice to have a Build.mk variable (perhaps WORKDIR_FILES?) that lists paths of files that should be copied into the workdir volume when it's created. This would allow copying files that are generated by build rules to the workdir volume. An example of how this could be useful is if a file required for the challenge to run that is created by some build rule is considered a secret. Therefore, the challenge author might wish to share the Docker image for the challenge without also revealing that secret generated file. One way to achieve this today would be to make the rule generate this file directly into the workdir/ folder in the project directory, but that's messy and requires .gitignore/.dockerignore shenanigans.