src-cli: add flag to print all output produced by containers

Created by: mrnugget

Based on user feedback (see this recent example in Slack) I think it makes sense to add a -log-output (or similarily named) flag to src-cli that would cause src-cli to print all output produced by the containers.

Without that, it's hard to debug why and when something went wrong inside a container.

It might even make sense to inject a set -x into the temporary scripts we create inside the container?