Signal handling for graceful shutdown

I use dotnet:3.0-appservice image for golang based custom handler on Linux. I want to handle signals such as SIGINT/SIGTERM on custom handler, but it seems that we cannot do it because of start.sh (PID 1) does not handle these signals.

At the moment, we have to wait 10 seconds and get exit code 137 (SIGKILL) when I stop the container. Not graceful.

So, I manage it with [domb-init](https://github.com/Yelp/dumb-init) as a workaround.

```Dockefile
[snip]
RUN wget -O /usr/local/bin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.5/dumb-init_1.2.5_x86_64
RUN chmod +x /usr/local/bin/dumb-init

ENTRYPOINT ["/usr/local/bin/dumb-init", "--"]
CMD ["bash", "/azure-functions-host/start.sh"]
```

Could you please consider implementation of handling logic on start.sh or publishing official guidelines? Thanks.