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>

doc/build/docker.rst

@@ -0,0 +1,14 @@
+GitLab CI / U-Boot runner container
+===================================
+
+In order to have a reproducible and portable build environment for CI we use a container for building in.  This means that developers can also reproduce the CI environment, to a large degree at least, locally.  This file is located in the tools/docker directory.  To build the image yourself
+
+.. code-block:: bash
+
+    sudo docker build -t your-namespace:your-tag .
+
+Or to use an existing container
+
+.. code-block:: bash
+
+    sudo docker pull trini/u-boot-gitlab-ci-runner:bionic-20200807-02Sep2020

doc/build/index.rst

@@ -9,4 +9,5 @@ Build U-Boot
    source
    gcc
    clang
+   docker
    tools