Handle SIGTERM event and add test/oa in .dockerignore

[Joxit]

Only master can handle SIGTERM signals and I use process.exit because the close function is only returned by the listen function.

resolves #35

[missinglink]

Nice!

Is it possible to avoid process.exit here?

[Joxit]

Yes it's possible but less understandable (I think). I added a new commit without process.exit.

It's a bit weird, when I use worker.kill('SIGTERM') we can't catch the SIGTERM event and the worker died instantaneously, so it equivalent to a process.exit.

I think this last commit is the best way to do a graceful shutdown.

[missinglink]

Tested via docker-compose and works for me 👍