Sbt run exits when run by script in a subprocess

At the moment I work on play with docker. The goal is to have lightweight container for development and deployment. The developer should not need to install a local version of the jdk and the deployment image should be as small as possible.
Actually the project is kind of ready but one thing does not work. Because I need a kind of filewatcher in the development container which copies files to a temporary memory volume sbt run is called in a bash script in a subprocess. It starts normally but than immediately shuts down without any interaction or signal sent to it.

I kind of tracked it down to a small bash script to test it with any sample play project:

#!/usr/bin/env bash
sbt "run 9000" &
wait "$!"
exit "$?"

From here on I don’t know how to go on. I tried to read the sbt documentation but did not find any clue on preventing this behaviour.

Is there any argument which can be passed? Is it a problem with play or with sbt? Should this be a bug ticket?

Thanks for your help.