Not sure how the --command flag works or is supposed to work.

This works for me, in that I get a running nginx with bash looping forever and printing 'bye'. kubectl run -it testnew --image=nginx -- /bin/bash -c "service nginx start && while true; do echo bye; sleep 10;done"

Instead of this special command, you probably want to create a tweaked image that runs a script on start. Easier to manage what is running and harder to lose the customizations.