tools: Integrate the Dockerfile used for CI

Integrate the Dockerfile from
https://source.denx.de/u-boot/gitlab-ci-runner.git

 as of
commit bc6130d572f1 ("Dockerfile: Remove high UID/GID") and introduce a
short rST on how to build the container.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>