Keep Dockerfile tidy

[jayvdb]

Due to aggressive deletion of unnecessary files, there are many explicit paths listed in the Dockerfile for deletion . They may not exist as the packages installed dynamically change.

At the moment, /usr/lib64/R/library/*/man/* no longer has any files, and should be removed.

To avoid scaring off newcomers, these shouldnt be build breakages, requiring immediate attention, but should be easy to find in the build logs and periodically remove.

[jayvdb]

I think we need a script which is shared between Travis and the docker , which has various sub-commands that can do various checks and cleanups.