util: Added cloudbuild_create_images.yaml

util: Added cloudbuild_create_images.yaml

This cloudbuild file is used to build and upload our Docker images
to our Google Cloud infrastructure. To run:

```
gcloud builds submit --config \
util/cloudbuild/cloudbuild_create_images.yaml
```

Change-Id: I812584284a3cf79aac244a3b04a0f316f4281c49
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30516
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>