Updated missed v6 docker image update

This will only have an effect when the version isn't set in the .env file and we have it in the .env.example but did still need updating as the fallback.

compose.yml

@@ -143,7 +143,7 @@ services:
 
   tinybird-sync:
     # Do not alter this without updating the Ghost container as well
-    image: ghost:${GHOST_VERSION:-5-alpine}
+    image: ghost:${GHOST_VERSION:-6.0.0-rc.0-alpine}
     command: >
       sh -c "
         if [ -d /var/lib/ghost/current/core/server/data/tinybird ]; then