use tini to avoid zombie processes

[lazka]

See #5

[lazka]

It's not clear if this helps, so closing for now.

If anyone ends up here and thinks otherwise, please speak up!

[jeremyd2019]

I tried to do a little investigation, but keep in mind I don't really deal with docker. What I saw is that in the current normal case, when using podman run, PID 1 is filled by the instance of bash running the msys2 script. It seems bash is reaping child processes. When using this PR, PID 1 is filled by tini, and PID 2 is the bash running the msys2 script.

However, if I attempt to reproduce what GHA does by doing
podman run -it --entrypoint /usr/bin/tail ghcr.io/msys2/msys2-docker-experimental -f /dev/null, PID 1 is still tail and I get zombie processes even with this change.