Expected startup times with container "enableImageStreaming": true

[jacksonwb]

When running a job with a fairly large image (few GBs) transition time from SCHEDULED to RUNNING takes about 6 mins.

When enabling container runnable image streaming with `enableImageStreaming` I don't really observe any difference in startup time. What is the expected improvement?

When describing the job the only change I observe is an extra label is added

```
labels {
  key: "goog-batch-managed-container"
  value: "enabled"
}
```

And the container runnable volumes string appends a `:false` to the supplied path strings.