Validate in CI that the Docker image can actually run

https://github.com/envoyproxy/nighthawk/issues/1312 found that the image published to https://hub.docker.com/r/envoyproxy/nighthawk-dev/tags was broken for everyone. It would not even have been able to start on the machine where it was built.

This is a way to check, with `docker run` picking up the newly built image:

```
ci/do_ci.sh docker
docker run envoyproxy/nighthawk-dev:latest nighthawk_client --help
```