README: Mention Embedded Containers Zephyr image

There are currently many complaints saying that the upstream Zephyr Docker
images are too large and contain many tools not required for downstream CI
operation.

Mention the Zephyr image provided by the Embedded Containers project,
maintained by Jonathan Beri who is an active Zephyr contributor, at the top
of the README file for the people looking for smaller images.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

README.md

@@ -8,6 +8,14 @@ This repository contains the Dockerfiles for the following images:
 - **Developer Image (_zephyr-build_):** includes additional tools that can be useful for Zephyr
   development.
 
+The Docker images hosted by this repository provide the "full" environment for upstream Zephyr CI
+testing and development, containing the toolchains for all supported architectures and various other
+tools for testing.
+
+If you are looking for a smaller image for downstream CI or local development targeting a specific
+architecture, consider using the [Zephyr image provided by the Embedded Containers
+project](https://github.com/embeddedcontainers/zephyr).
+
 ## Developer Docker Image
 
 ### Overview