The 'runs.image:' argument was using `Dockerfile`. This caused
a Docker build of the trufflehog image every time someone
used this action.
I've changed the argument to use the already published Docker image.
This will speed up the workflows considerably, since it pulls the
image that is already built and available on GitHub's image storage.
* Log explanation of non-zero exit with found results
* Change exit code and add documentation
* Adjust exit code handling
* Make action fail on found results
* Use new overseer
* Improve wording
* Update README.md
