Hm… Typically you would have a CMD directive at the tail end of the Dockerfile…
Try looking in the logs (fly logs) for a line of the form Preparing to run: `foo` as root. My guess is that the foo command in your particular case is just an inherited placeholder that is exiting immediately. (But I’m definitely not one of the local Node experts.)
(Maybe look at some of the other Node users’ Dockerfiles in the forum archives and see what they tend to use, 
.)