To create a Cloud Run new job :

gcloud beta run jobs create JOB_NAME --image IMAGE_URL OPTIONS

To update existing job :

gcloud beta run jobs update JOB_NAME

If you want a single command to handle the creation and update at the same time, you can develop your own Shell script, example :

#!/usr/bin/env bash

set -e
set -o pipefail
set -u

export JOB_NAME=my_job

res=$(gcloud beta run jobs describe $JOB_NAME --region=europe-west1 || echo "NOT_EXIST")

echo "#######Result : $res"

if [ "$res" = "NOT_EXIST" ]; then
  echo "Creating your job..."
  gcloud beta run jobs create $JOB_NAME
else
  echo "Updating your job..."
  gcloud beta run jobs update $JOB_NAME
fi

Before you push image to GAR, you can docker tag your docker image with latest

In Cloud Run Job, just select the image with the latest tag

So that every time Cloud Run Job be triggered, it would use the latest image