Allow viewing/accessing logs of sidecar containers

Feature request: It would be helpful for us to be able to access logs for sidecar containers that agent-stack-k8s starts, to help with troubleshooting or see status as the job is running.

Ideally this would be accessible in the Buildkite UI in real-time, but at the very least being able to archive them as job artifacts or to access them with a `buildkite-agent` subcommand would be better than nothing. Thanks!