automation/dockers: add to README how to rebuild all containers

Document in the README how to rebuild all containers.  This is helpful when
populating a local docker registry for testing purposes.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

automation/build/README.md

@@ -108,6 +108,13 @@ env CONTAINER_NO_PULL=1 \
 make -C automation/build opensuse/tumbleweed-x86_64 PUSH=1
 ```
 
+To rebuild all containers the `all` make target can be used, with or without
+the `PUSH` environment variable:
+
+```
+make -C automation/build all PUSH=1
+```
+
 [BuildKit]: https://docs.docker.com/build/buildkit/
 [registry]: https://gitlab.com/xen-project/xen/container_registry
 [registry help]: https://docs.gitlab.com/ee/user/packages/container_registry/