point out that ENV DEBIAN_FRONTEND will persist, so its not recommended

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #4035 from SvenDowideit/dont-env-debian-frontend

point out that ENV DEBIAN_FRONTEND will persist, so its not recommended