Added graceful timeout in seconds

[SimonSimCity]

Added the option app.gracefulTimeout to decide many seconds a graceful shutdown is allowed to take - which is - how long docker should wait from calling docker stop (which triggers SIGINT) to calling docker rm -f (which triggers SIGTERM).

I don't know at which place the documentation can be updated, so I've left this task open.

The default value of app.gracefulTimeout is 10 and is in seconds.

As of how I see it, the other containers don't need a graceful shutdown, or do they? If yes, we need to take into account that e.g. nginx uses a different signal for a graceful shutdown than SIGINT - this just as a small note in case it's required.

This is untested though ...