runner: drop volume name prefix, whitelist in valid_volumes

runner/compose.yml

@@ -28,12 +28,17 @@ services:
 
 volumes:
   cargoxx-cache-store:
+    # Explicit name disables compose's project-prefix so spawned job
+    # containers (which don't know about compose) can reference the
+    # same volume by the same name.
+    name: cargoxx-cache-store
     driver: local
     driver_opts:
       type: none
       o: bind
       device: "${PWD}/cache/store"
   cargoxx-cache-keys:
+    name: cargoxx-cache-keys
     driver: local
     driver_opts:
       type: none