Update dockerfile to accept sigint more gracefully

[mcascone]

When you have to kill a bitops run on the command line, with ctrl-c, it just hangs. It would be nice if it would exit gracefully.

I believe this can be done by adding STOPSIGNAL SIGINT to the dockerfile, or by running the main process inside a dummy process that catches the signal and exits.

likely resource: tini entrypoint

(ported into bitops from https://github.com/bitovi/operations-repo-examples-internal/issues/6)