Simplest "CMD" line to keep a container up

I’m doing some python development in a general linux (debian) container & don’t yet have an application to run. What’s the smallest/lightest “CMD” line I can add in my Dockerfile to keep the container running?

For now, I’m using nginx, as I can copy 2 lines off another Dockerfile to do that, but nothing in my present container needs it.

['tail', '-f', '/dev/null']

If you just want to work with an existing image you can even add this as command parameter to the service in your compose file