Rename rockylinux9 build directory to rocky9

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -9,20 +9,20 @@ Each build script automatically selects Docker or Podman:
 ```sh
 ./build-all.sh
 ./debian13/build.sh
-./rockylinux9/build.sh
+./rocky9/build.sh
 ./alpine3.24/build.sh
 ```
 
 Use environment variables to select the container engine or override image names:
 
 ```sh
-CONTAINER_ENGINE=podman IMAGE_NAME=my-build-env ./rockylinux9/build.sh
+CONTAINER_ENGINE=podman IMAGE_NAME=my-build-env ./rocky9/build.sh
 ```
 
 ## Run
 
 ```sh
 docker run --rm -it debian13-source-stack
-docker run --rm -it rockylinux9-source-stack
+docker run --rm -it rocky9-source-stack
 docker run --rm -it alpine3.24-source-stack
 ```