Add option to download docker containers

[mugithi]

Code sample

if [ -z $GCE_IMAGE_VERSION && $XLA_DOCKER_CONTAINER == 'true' ]
then 
    docker pull docker pull gcr.io/tpu-pytorch/xla:nightly_$GCE_IMAGE_VERSION
fi 

[mugithi]

Example

google-terraform-pytorch-tpu/env_setup/scripts/setup_mig.sh

Line 72 in 13ce037

#docker pull gcr.io/tpu-pytorch/xla:nightly_3.6