Show some ENV / local updated baseimage tricks that use an apt-cacher-ng proxy to make debian based installations instant

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
implement pharvey's suggestions

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)