Fast updating an yaml file inside the container

I’m trying to use the --file-local through console and/or machine update to simple copy to /app/ but getting error:

fly console --file-local "/app/config.yaml=config.yaml"
Error: failed parsing files: failed to read file-local: guest path, /app/config.yaml, must be absolute

For reference I used this: VM for one-off scripts on Rails app - #5 by leo-frdm