Add a in container exec based health check.

I think that regexing is something we can delegate - if the user needs to regex, run command = [ "sh", "-c", "cmd | grep OK"].

I'd rather start very simple - if we find this to be cumbersome, we can always ADD regexing, but once we have added it we have to answer other requests like "check my status code and allow X, Y, and Z as success" and we have to answer why HTTP checks don't do a regex, too.  Or TCP sockets.

We should keep those things in rough feature parity until we have compelling reasons to deviate.  IMO.