Update dockerfile to accept sigint more gracefully

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](https://github.com/hexops/dockerfile#use-tini-as-your-entrypoint)

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

Start with the repository's Dockerfile and entrypoint configuration, then review the linked tini entrypoint resource and compare it with the suggested STOPSIGNAL SIGINT approach. Done means a BitOps run started from the command line exits gracefully rather than hanging when interrupted with Ctrl-C.

Written by the indexing model from the issue text.