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)
All caps variables in normal bash should be avoided not to accidentally collide with environment variables.

Docker-DCO-1.1-Signed-off-by: Rovanion Luckey <rovanion.luckey@gmail.com> (github: Rovanion)
fixes suggested by James

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #4143 from SvenDowideit/apt-cacher-example

Show some ENV / local updated baseimage tricks that use an apt-cacher-ng...
Merge pull request #4526 from Rovanion/master

All caps variables in normal bash should be avoided not to accidentally collide with environment variables.