I think you already solved this problem, since you asked it on September 2023.
Anyway, I solved by using socat to forward any requests come to 0.0.0.0:3000 to localhost, on the mongosync default port.
My dockerfile is similar to the following one:
RUN apt add socat
...
CMD ["/bin/sh", "-c", "mongosync & socat TCP-LISTEN:3000,fork TCP:127.0.0.1: 27182"]
```