Switch single platform via env var

extensions/molecule/default/molecule.yml

@@ -8,16 +8,8 @@ driver:
     ansible_connection_options:
       ansible_connection: docker
 platforms:
-  - name: rockylinux9
-    image: geerlingguy/docker-rockylinux9-ansible:latest
-    command: "/sbin/init"
-    volumes:
-      - /sys/fs/cgroup:/sys/fs/cgroup:ro
-    privileged: true
-    pre_build_image: true
-
-  - name: fedora39
-    image: geerlingguy/docker-fedora39-ansible:latest
+  - name: instance
+    image: geerlingguy/docker-${MOLECULE_DISTRO:~rockylinux9}-ansible:latest
     command: "/sbin/init"
     volumes:
       - /sys/fs/cgroup:/sys/fs/cgroup:ro