fix: ensure leading space when injecting --entrypoint launcher

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`).

plugins/builder-pack/docker-args-process-deploy

@@ -20,7 +20,7 @@ trigger-builder-pack-docker-args-process-deploy() {
 
   if [[ "$inject_launcher" == "true" ]]; then
     # without this, the command and arguments are passed as args to the default process type
-    output="--entrypoint launcher "
+    output=" --entrypoint launcher"
   fi
 
   echo -n "$STDIN$output"