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>
diff --git a/doc/build/index.rst b/doc/build/index.rst
index 5f90f95..69952f9 100644
--- a/doc/build/index.rst
+++ b/doc/build/index.rst
@@ -9,4 +9,5 @@
    source
    gcc
    clang
+   docker
    tools