added signal handling and db2diag displaying

[bmanojlovic]

entrypoint script has been extended so that it correctly shutdown database when is called trough

docker stop <container_name>

Of course docker kill will not do shutdown procedure but neat thing with signal handling is that now

docker kill -s USR1 <container_name>

will provide instance restart on demand without tearing down container. Logs of db2diag can be seen using default logging behavior:

docker logs -f <container_name>

[wzymaster]

@bmanojlovic thanks very much and let me know if there is anything else that I can follow up.