docs(ct-base): add notes about multiarch builds IQSS#8932

Addin description on requirements to build cross platform
added as subsection of the build instructions seemed valuable.

doc/sphinx-guides/source/container/base-image.rst

@@ -86,6 +86,22 @@ its sources plus uncached scheduled nightly builds to make sure security updates
 *Note:* For the Github Action to be able to push to Docker Hub, two repository secrets
 (DOCKERHUB_USERNAME, DOCKERHUB_TOKEN) have been added by IQSS admins to their repository.
 
+Processor Architecture and Multiarch
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Using Maven Docker Plugin's *BuildX* mode, the base container image provides support for multiple CPU architectures
+from the same image.
+
+Building the image via ``mvn -Pct package`` or ``mvn -Pct install`` as above will only build for the architecture of
+the Docker maschine's CPU.
+
+Only ``mvn -Pct deploy`` will trigger building on all enabled architectures (which are AMD64 and ARM64 at this time).
+Yet, to enable building with non-native code on your build machine, you will need to setup a cross-platform builder.
+
+On Linux, you should install `qemu-user-static <https://github.com/multiarch/qemu-user-static>`__ (preferably via
+your package management) on the host and run ``docker run --rm --privileged multiarch/qemu-user-static --reset -p yes``
+to enable that builder. The Docker plugin will setup everything else for you.
+
 
 
 Tunables
@@ -290,8 +306,6 @@ you can read about those in a few places like https://developers.redhat.com/arti
 https://www.eclipse.org/openj9/docs/xxusecontainersupport, etc. The other memory defaults are inspired
 from `run-java-sh recommendations`_.
 
-*Note: the build process used the newer ``buildx`` feature of Docker to provide multiarch images.*
-
 
 
 .. _Pre/postboot script docs: https://docs.payara.fish/community/docs/Technical%20Documentation/Payara%20Micro%20Documentation/Payara%20Micro%20Configuration%20and%20Management/Micro%20Management/Asadmin%20Commands/Pre%20and%20Post%20Boot%20Commands.html