Allow docker network to be specified using the --network flag

[pyljain]

I am trying to invoke a Kpt function within a Cloud Build pipeline, the function runs in a docker container within the docker container that gets created for the cloud build step. The function needs to be able to call GCP APIs and hence needs access to ADC (Application Default Credentials). In a cloud build step, in order for the function's docker container to get access to ADC, the docker container needs to be attached to the cloudbuild local docker network. I need the ability to specify the network name when invoking kpt fn run in a cloud build step. e.g.:

kpt fn run sample/manifests/ \
  --network=cloudbuild \
  --image myimage:dev