Add rocky9-system image using distribution libraries

Install libraries from Rocky Linux 9, CRB and EPEL instead of building
from source; packages missing from those repositories are skipped.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -10,6 +10,7 @@ Each build script automatically selects Docker or Podman:
 ./build-all.sh
 ./debian13/build.sh
 ./rocky9/build.sh
+./rocky9-system/build.sh
 ./alpine3.24/build.sh
 ```
 
@@ -26,10 +27,13 @@ PREFIX=/opt/stack ./debian13/build.sh
 PREFIX=/opt/stack ./rocky9/build.sh
 ```
 
+The `rocky9-system` image uses the libraries shipped by Rocky Linux 9 (BaseOS, AppStream, CRB and EPEL) instead of building them from source. Libraries that those repositories do not provide, such as Folly and the other Meta libraries, are simply left out.
+
 ## Run
 
 ```sh
 docker run --rm -it debian13-source-stack
 docker run --rm -it rocky9-source-stack
+docker run --rm -it rocky9-system-stack
 docker run --rm -it alpine3.24-source-stack
 ```