update mockotlpserver Dockerfile to support passing `docker run` args to the CLI

It would be nice to be able to pass CLI arguments to mockotlpserver when running it via the published Docker image.
CUrrently that doesn't work:

```
docker run --rm -it -p 4317:4317 -p 4318:4318 --name mockotlpserver \
ghcr.io/elastic/elastic-otel-node/mockotlpserver --help
```

That `--help` arg is going to `node` rather than to the mockotlpserver `lib/cli.js`. Low priority.