please, for the love of Docker, do. not. use. symlinks

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #3987 from SvenDowideit/warn-against-symlinks

please, for the love of Docker, do. not. use. symlinks