The trigger output started with `--entrypoint` and relied on the caller
to provide a trailing space. `scheduler-deploy-process-container` does
that, but `scheduler-run` does not when `DOKKU_TRACE` is unset, so the
flag concatenated onto the previous arg as `--env=KEY--entrypoint`,
leaving `launcher` as the image positional argument and causing
`docker container create` to fail with `Unable to find image 'launcher:latest'`.
Match the leading-space convention used by the other docker-args triggers
(`config/docker-args-run`, `builder-herokuish/docker-args-run`).
If no Procfile is set, we'll leave the entrypoint alone, resulting in the default launcher for the web process (or whatever pack decided was default) executing.
Closes#5753
