No output when running `gcloud` command

Running a `gcloud` command in my build does not produce any output.

Snippet of build script in command block:
```
#!/usr/bin/env bash

echo "Deploying $IMAGE to $ENVIRONMENT"

gcloud compute instances update-container ${INSTANCE} \
--container-image ${IMAGE} \
--container-env-file ${ENV_FILE} \
--zone ${ZONE}

echo "Finished deploying $IMAGE to $ENVIRONMENT"
```

Output in Buildkite web UI:
```
Running script | 50s
  | $ ./deploy.sh
  | Deploying gcr.io/my-project/image:dev to dev
  | Finished deploying gcr.io/my-project/image:dev to dev
```

When I SSH into the agent and run the commands:

```
[buildkite-agent@ip-10-0-1-244 skepsi]$ gcloud compute instances update-container my-dev-instance --container-image gcr.io/my-project/image:dev --container-env-file env-dev --zone australia-southeast1-a
 
Updating specification of container [my-dev-instance]...done.
Stopping instance [my-dev-instance]...done.
Starting instance [my-dev-instance]...done.
[buildkite-agent@ip-10-0-1-244 skepsi]$
```

My guess is the `gcloud` command is adding a text-based spinner on the end of each line which Buildkite can't pick up?

Start by reproducing the deploy.sh command in a Buildkite job and compare its web UI output with the same gcloud command run over SSH. Investigate how the agent captures command output; done means the gcloud progress lines appear in the Buildkite log while the deployment still completes.

Written by the indexing model from the issue text.