Create docker/README.md, which documents the use of the docker/
folder. This will make it easier to find documentation on how the docker
images are built. This guide was originally found on the docker hub page
of savoirfairelinux/ring-ubuntu32 and was not under source control.

Change-Id: I8b8ab6179b6e1ba36974437a16463821f42f2a84
Reviewed-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>