doc: use docker to build documentation

docker (container) is another way to build the documentation and fortunately
there is already a docker image (sphinxdoc/sphinx) with sphinx so we can use
it to generate the documentation.

Change-Id: I06b0621cd7509a8279655e828680b92241b9fde4
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>

docs/getting_started/docs-build.rst

@@ -67,7 +67,7 @@ Output from the build process will be placed in:
 
 ::
 
-   docs/build/html/
+   docs/build/html
 
 We also support building documentation in other formats. From the ``docs``
 directory of the project, run the following command to see the supported
@@ -79,6 +79,31 @@ top-level Makefile for |TF-A| itself.
 
    make help
 
+Building rendered documentation from a container
+------------------------------------------------
+
+There may be cases where you can not either install or upgrade required
+dependencies to generate the documents, so in this case, one way to
+create the documentation is through a docker container. The first step is
+to check if `docker`_ is installed in your host, otherwise check main docker
+page for installation instructions. Once installed, run the following script
+from project root directory
+
+.. code:: shell
+
+   docker run --rm -v $PWD:/TF sphinxdoc/sphinx \
+          bash -c 'cd /TF && \
+          pip3 install plantuml -r ./docs/requirements.txt && make doc'
+
+The above command fetches the ``sphinxdoc/sphinx`` container from `docker
+hub`_, launches the container, installs documentation requirements and finally
+creates the documentation. Once done, exit the container and output from the
+build process will be placed in:
+
+::
+
+   docs/build/html
+
 --------------
 
 *Copyright (c) 2019, Arm Limited. All rights reserved.*
@@ -86,3 +111,5 @@ top-level Makefile for |TF-A| itself.
 .. _Sphinx: http://www.sphinx-doc.org/en/master/
 .. _pip homepage: https://pip.pypa.io/en/stable/
 .. _Dia: https://wiki.gnome.org/Apps/Dia
+.. _docker: https://www.docker.com/
+.. _docker hub: https://hub.docker.com/repository/docker/sphinxdoc/sphinx