Resolve "Docker containers are left running when Jenkins jobs are killed/aborted"

Replace lb-docker-run with docker logs -f $(docker run ...) and add an EXIT trap to kill the container when the Jenkins job exits.

This solves a problem (#9 (closed)), but it introduces some annoyances:

• changes to lb-docker-run are not picked up
• the mock mode is broken

I'm not planning to fix them as we will first switch to a new system.

Closes #9 (closed)