Document Docker build

The repository README doesn't currently mention Docker, but this is the
easiest way to build the OpenCL-Docs specs, as opposed to manually
configuring some quite dated ruby dependencies.

This PR updates the README to alert to developers that Docker is
available help them build the project.

It is worth noting that the build instructions for both
[Vulkan BUILD.adoc](https://github.com/KhronosGroup/Vulkan-Docs/blob/main/BUILD.adoc#khronos-provided-docker-image)
and
[SYCL README](https://github.com/KhronosGroup/SYCL-Docs#building-using-the-khronos-docker-image)
only document building the project using the docker image. We should
consider whether we want a larger rewrite of the OpenCL REAME to only
describe building the repo in terms of Docker, but for now the PR
makes an incremental improvement.

Author: EwanC

README.adoc

@@ -73,8 +73,16 @@ We intend to maintain a linear history on the GitHub `main` branch.
 [[building]]
 == Building The Specifications and Reference Pages
 
-If you have installed all <<depends,required toolchain components>>, you
-should be able to build both HTML and PDF outputs for all specifications by:
+After cloning the repository you should install all of the <<depends,required toolchain components>>.
+The recommended way to do this is by using the Khronos provided Docker image
+`khronosgroup/docker-images:asciidoctor-spec` from DockerHub.
+The Dockerfile specifying this image can be found at
+https://github.com/KhronosGroup/DockerContainers[KhronosGroup/DockerContainers]
+if you need to build a modified or layered image. Due to local and CI caching problems when this image
+is updated on dockerhub, the CI uses the SHA256 of the latest image update, rather than the image name.
+
+Once you have installed all the dependencies you should be able to build both HTML
+and PDF outputs for all specifications by:
 
     $ make