When run in the background, remove the container in "builder_stop",
this should make it easier to print logs between starting and
stopping the container.
The run-builder script now takes a command line argument, which is
either
start - run the container in the background
stop - stop the running container
fg - run the container in the foreground (old behavior)
This should prove useful in CI.
Simple start script for the builder. It is not integrated in
run-koji-container.sh because here in this repo, the builder
might be restarted often when working on the builder plugin.