Forward signals from entrypoint script for custom docker builds

[augray]

When a docker image is built using the docker builder, it uses an entrypoint bash script defined here. If the k8s pod this is running in receives a signal (ex: SIGTERM), the entrypoint.sh process will receive the signal, but the python interpreter it spawned will not. We should change that so that the script forwards the signal, and exits with the exit code of its child.