Set the default Docker command

The simplest way to add this worker to an instance, in order to start processes, is to use the frontend. The 1.6.1 upgrade notes mention the CLI in passing but mostly document how to do it with the frontend.

To properly add this worker, you need to set {"docker": {"command": "worker-init-elements"}} as the WorkerVersion's configuration, otherwise the container will do nothing and exit immediately. It runs the Python REPL with no standard input, so it just quits without any error. If you forgot the configuration when adding the version, you get stuck and confused. The S3 import worker does not have this issue, as it has a default command set.

Please set a default command in this worker's Dockerfile.